Abstract
Statistical thresholds occur when the changes in the relationships between a response and predictor variables are not linear but abrupt at some points of the predictor variable values. In this paper, we defined a piecewiselinear regression model which can detect two thresholds in the relationships via changes in slopes. We developed the corresponding Bayesian methodology for model estimation and inference by proposing prior distributions, deriving posterior distributions, and generating posterior values using Metropolis and Gibbs sampling algorithm. The parameters in our model are easy to understand, highly interpretable, and flexible to make inferences. The methodology has been applied to estimate threshold effects in housing market pricing data in two cities  Kamloops and Chilliwack  in British Columbia, Canada. Our findings revealed that the implementation of changes in the government property tax policies had threshold effects in the market price trend. The proposed model will be useful to detect threshold effects in other correlated time series data as well.
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1 Introduction
Purchase of residential housing constitutes the largest expenditure for most households [10]. For that reason, unlike other commodities purchased by households, home purchase is considered an investment item in national income accounting. The housing market provides many important linkages for various sectors of the economy generating significant impact on the level and rate of change in aggregate income of an economy, most often measured by gross domestic product (GDP) [27].
As with all commodities in the market, housing prices are determined by the forces of demand and supply [17, 32]. However, specific factors on demand and supply differ depending on the nature of the commodity in question. While at the general equilibrium level all markets and all factors influencing individual market are interrelated, some of the most relevant determinants of housing prices on the demand side are: interest rates, the state of the economy, demographics including immigration, availability of mortgage financing, affordability of housing, expectations, and government policies and legislation including tax incentives, tax deductibility and subsidies [16]. On the supply side, the important determinants are: number of active listings reflecting availability of homes for sale, new home construction reflecting increase in the stock of housing, and land use and zoning reflecting the availability of land for different types of buildings permitted [18].
A wide array of studies has attempted to capture the impacts of various demand and supply side factors on housing prices. Nistor and Reianu [33] examined the effect of immigration on housing prices in the ten largest metropolitan areas in Ontario, Canada. Using quarterly data for Canada, Hossain and Latif [24] found that housing price volatility is affected significantly by GDP growth rate, housing price appreciation rate and the rate of inflation. Another study of the factors influencing home prices in the greater Toronto area in Canada [6] identify a strong economy, low interest rates and favourable mortgage insurance rules on the demand side, and limited supply of housing stock on the supply side as contributing factors for price increases.
The housing markets of large Canadian cities have been attractive places of investment for foreign buyers [11]. It has not been more pronounced anywhere than in Vancouver which has attracted a large inflow of foreign buyers due to its proximity to the AsiaPacific region, mild weather and natural beauty [12, 23]. Limited supply of land and strong demand resulted in a rapid increase in real estate prices which prompted for calls from the public to curtail the demand from foreign buyers [22, 25]. In response to that, the provincial government enabled the cities and municipalities to impose an additional property transfer tax on foreign buyers for purchases of residential properties. The tax was first introduced in Vancouver and some other cities soon followed suit\(^{1}\). This additional tax currently stands at \(20\%\) of the fair market value of the property. Our study attempts to fill a void in literature by examining the impact of change in government policy on the housing prices of the Province of British Columbia, Canada. If all else remains unchanged, we expect that the imposition of the additional foreign buyer’s tax in one city would impact the housing prices of other cities in the province by shifting demand away from the city with the tax, to cities without.^{Footnote 1}
We propose a statistical model to measure association between housing price and time demarcated by thresholds. Two variables are considered to be related to each other when there is a change in the distribution of one variable for a change in the other variable [15]. In a regression set up, the variables are considered to be related if the conditional mean of the response variable changes for a change in the predictor [14]. The variables are considered to be related with threshold effect when the slope of the regression relationship changes abruptly at a certain level of the predictor variable, which is known as the breakpoint [44]. Estimation of breakpoint is very popular in many applications. In environmental ecology, for example, the piecewiselinear regression model (PLRM) is often used as a tool to calculate environmental thresholds resulting from human induced disturbances to nature [4, 19, 45]. In Business and Economics, estimation of breakpoints in the housing market is known as the estimation of structural breaks [3, 9]. Our goal in this paper is to use the PLRM to estimate the breakpoints in housing prices trends resulting from the adoption and changes in government policies.
A piecewiselinear regression model (PLRM) with one breakpoint represents two linear lines with differing slopes demarcating the data into two segments [4]. Muggeo [31] proposed a PLRM (also know as segmented regression model) to estimate breakpoints using classical statistical method. Toms and Lesperance [44] developed several PLRMs where two linear lines are connected at the breakpoint using various mechanisms. Examples of estimating a single breakpoint can be found at [19, 38, 44, 45]. A PLRM with two breakpoints has three slopes, defined at three segments of the data, where two slopes are connected at a single breakpoint. Tomal and Ciborowski [42] developed a PLRM with two breakpoints and applied the model to estimate environmental thresholds from human induced disturbances to nature. All of the above studies are based on either classical statistical or quantile regression methods.
There is considerable interest in developing Bayesian models for the detection of statistical thresholds via the changes in slopes. Qian et al. [35] proposed a Bayesian hierarchical model (BHM) for the detection of environmental threshold effects of total phosphorus on macroinvertebrate composition in wetlands. Liang et al. [28] developed a Bayesian change point model to estimate threshold effects of nutrientsphytoplankton relationship in lakes. Ouyang et al. [34] extended BHM to improve the estimates of buildup areas from night time light across globally distributed cities. Bucci et al. [5] proposed a Bayesian segmented regression model and applied their method to forensic age estimation using the assumption that the data points were independent to each other. All of the above Bayesian models were developed to detect a single threshold via the changes in slopes using a PLRM. In this paper, we develop a Bayesian PLRM to detect two thresholds via changes in slopes.
Chien [9] used a Lagrange Multiplier unit root test [26] to examine the issue of whether regime changes had broken down the stability of the ripple effect in Taiwan’s housing market. Ravazzolo et al. [36] utilized Bayesian model averaging, an ensemble method, to estimate structural breaks in the regression parameters, uncertainty about the inclusion of forecasting variables, and uncertainty about parameter values with application in the stock market. Vizek and Posedel [47] used threshold autoregressive (TAR) and momentum TAR (MTAR) models, defined thresholds in terms of the changes in the error term, to test if housing prices in the United States, United Kingdom, Spain and Ireland were characterized by threshold effects. On the other hand, Begiazi and Katsiampa [3] used return [\((ln(p_t)  ln(p_{t1})) \times 100\)] and the GARCH model to detect structural breaks in housing prices in the United Kingdom. Their transformation and the GARCH model are useful for examining volatility in price, breaks in variancecovariance, and conditional heteroscedasticity. Unlike Begiazi and Katsiampa [3], the objectives of our paper are proposing a Bayesian model for time series data to estimate breaks in linear trend and interpreting the linear relationships estimated from the slopes of the piecewise linear model. Therefore, our methodology and objectives are different than the methodology and objectives of the studies mentioned above. Moreover, we have transformed our data using “return” as in Begiazi and Katsiampa [3], and found that there is no noticeable breaks in volatility, variancecovariance, and conditional heteroscedasticity. Additionally, the transformation using return causes to lose one data point, especially for the first month, which might appear crucial in statistical estimation and model building when the amount of data is limited.
Time series data is collected over time and, as a result, the successive data points are correlated as opposed to independent. Djafari and Féron [30] used a Bayesian method for change point detection in variables that are collected over time. Their method can be used to detect change points via the distribution of the variables. Ruggieri [37] used a Bayesian approach for the detection of change points in climatic records through the probability density function. Instead of using a regression model that is defined for the entire data series, the author used chain rule of probability theory by multiplying the conditional and marginal probabilities to obtain the joint probability. On the other hand, we have defined a piecewiselinear regression model with two breakpoints via the changes of slopes which is defined for the entire set of data. Unlike Bucci et al. [5], we have extended the Bayesian piecewise linear regression model to time series data where the errors are correlated. Instead of assuming independence of data points, we proposed an autoregressive correlation structure of the residuals which allowed us to calculate correlations between successive data points. We further showed how these correlations can be estimated from MCMC samples using Metropolis algorithm. Note that similar classical statistical models developed for time series data to capture breakpoints are available in the literature [1, 39, 40, 46]. Our proposed model is a Bayesian and extended version of the standard classical models. The beauty of our model lies in its simplicity, less involvement of model parameters, and evidencebased intuitive assumptions. The main strengths of our paper are the interpretability of the model parameters (slopes and breakpoints), and simple and straightforward statistical inference procedures.
The proposed Bayesian piecewiselinear regression model has been applied to data of two housing markets—(1) Chilliwack, BC, and (2) Kamloops, BC. We hypothesize that the changes in government policies in large cities, such as imposition of the new nonresident property transfer tax, cause threshold effects on home prices in nearby cities. We also hypothesize that when the government changes policies such as imposition of new nonresident property transfer tax in the smaller cities, it also causes similar ripple threshold effects in nearby cities of similar size.
Therefore, the first objective of our paper is geared by the application for the detection of threshold effects of the introduction of government’s tax policy changes in Vancouver, British Columbia, Canada on housing prices in two similar sized cities in the province (similar in terms of population, business capacity and size of universities). The second objective is to extend the Bayesian piecewise linear regression model (segmented regression model) for the detection of thresholds in time series data. Our findings showed that the changes in government tax policies caused significant threshold effects in housing prices.
The model building procedure is summarized as follows. Firstly, we visualize the data to observe the type of relationships between variables and define the model. The model is finalized by adaptive imposition of the appropriate correlation structure evident in the data points. We then define appropriate prior distributions for the model parameters, and derive their posterior distributions. We propose the Metropolis and Gibbs sampling algorithm to generate Markov Chain Monte Carlo (MCMC) samples from the posterior distributions. Lastly, we propose a twostep procedure to determine appropriate proposal distribution for the parameters of interest.
2 Housing price/time series data
We collect time series data of monthly average home prices for two small cities in British Columbia—Chilliwack, which is about 100 km east to metropolitan Vancouver, and Kamloops, which is about 350 km northeast of Vancouver. These two cities are of similar size in terms of landmass, business capacity and population, and each is the home to one similarsized university. The data are collected from January 2011 to July 2020 for a total of 115 months from the monthly statistical report of the Canadian Real Estate Association (CREA) (https://creastats.crea.ca/enCA/). In our data, the monthly average home prices are the average of all types of houses such as detached and semidetached houses, townhouses, and condos.
3 Methods
Before we propose the model, we introduce the autocorrelation function (ACF) of the monthly average home prices in Chilliwack (left panel) and Kamloops (right panel), British Columbia (BC) against time lag (months) in Fig. 1. We can see that the monthly home prices are highly correlated. Even at the time lag of month 21, the autocorrlation is above 0.40 and highly statistically significant. This observation leads us to propose a model with positively correlated errors as defined below.
3.1 The model
Let Y and x be the average monthly home price and the linear trend variable, in month, respectively. We define our model as:
where \(\theta _4\) and \(\theta _5\) are the first and second breakpoints, respectively, and
for \(j \in \{4, 5\}\) and \(\varvec{\epsilon } = \left( \epsilon _1, \epsilon _2, \ldots , \epsilon _n\right) ^T \sim \text {Multivariate Normal}({\mathbf {0}}, \Sigma )\). The linear trend part of our proposed model deals with nonstationarity in time series data modeling. In this formulation, \(\theta _1\), \(\theta _1 + \theta _2\) and \(\theta _1 + \theta _2 + \theta _3\) are the slopes in the first, second and third segments of the model demarcated by the thresholds \(\theta _4\) and \(\theta _5\), respectively. When \(\theta _2\) and \(\theta _3\) are 0, the thresholds \(\theta _4\) and \(\theta _5\) are statistically insignificant and, thus, there is no threshold via the changes in slope. The beauty of this model lies in the easy interpretation of the linear trends via the slopes.
Given the conditional mean of the response variable against linear trend of time denoted as \(E\left( {\mathbf {Y}} {\mathbf {x}}, \varvec{\theta }, \Sigma \right) = \theta _0 + \theta _1 {\mathbf {x}} + \theta _2 ({\mathbf {x}}  \theta _4) I ({\mathbf {x}}  \theta _4) + \theta _3 ({\mathbf {x}}  \theta _5) I ({\mathbf {x}}  \theta _5) \), the conditional distribution of the response variable can be written as
where \(\Sigma \) is the variancecovariance matrix of the vector of error term. From the autocorrelation function plot of the average home price, we consider that the error terms are not independent, but temporally autocorrelated. As a result, the structure of the variancecovariance matrix \(\Sigma \) needs to reflect positive autocorrelation between sequential observations. As the autocorrelation gradually decreases over time, we consider the following firstorder autoregressive structure for the variancecovariance matrix of the error term:
where \(\rho \) is the correlation between successive error terms \(\epsilon _t\) and \(\epsilon _{t+1}\). Under this covariance structure, the variance of \(\epsilon _t\) is \(\sigma ^2\), and the autocorrelation between \(\epsilon _t\) and \(\epsilon _{t + d}\) is \(\rho ^d\), which decreases as the timelag d grows larger.
3.2 The sampling distribution of data
Let \(\varvec{\theta } = \left( \theta _0, \theta _1, \theta _2, \theta _3, \theta _4, \theta _5\right) ^T\) be the vector of regression coefficients. Then the sampling distribution of data is defined as
where MVN stands for the multivariate normal distribution with mean vector \(E\left( {\mathbf {Y}} {\mathbf {x}}, \varvec{\theta }, \Sigma \right) \) and variancecovariance matrix \(\Sigma \).
3.3 Prior distributions
The prior distribution for the error variance is considered as InverseGamma as following
where the prior mean is \(E(1/\sigma ^2) = 1/ \sigma _0^2\) with strength of prior belief \(\nu _0\). Here, larger values of \(\nu _0 > 0\) indicate stronger prior belief and vice versa.
The prior distribution for \(\varvec{\theta }\) is considered as MultivariateNormal
with prior mean \(\varvec{\mu }_0\) and variancecovariance matrix \(\Sigma _0\).
As noted before, we consider that the successive observations are positively correlated, and use a flat prior for correlation coefficient \(\rho \) on the range from 0 to 1:
where Unif stands for the density of Uniform distribution defined over [0, 1].
3.4 The posterior distributions
The posterior distribution for the error variance \(\sigma ^2\) is obtained by combining the sampling distribution (Eq. 2) and the prior distribution of \(\sigma ^2\) (Eq. 4) as following:
where the sum of squares of errors is expressed as
Unfortunately, the posterior distribution for \(\varvec{\theta }\) has no closed form expression. We write
where MVN stands for the Multivariate Normal distribution.
As like the posterior distribution for \(\varvec{\theta }\), the posterior distribution for \(\rho \) has no closed form expression. Thus, we write
where Unif stands for the Uniform distribution defined over [0, 1].
3.5 Metropolis and Gibbs sampling algorithm
As we do not have a closed form expression for some of the posterior distributions, we cannot use Monte Carlo method or Gibbs sampling to generate posterior values. Instead, we propose the following Metropolis and Gibbs sampling algorithm to generate samples from the full posterior distribution of the parameters. Here, we have fused the Metropolis and Gibbs sampling algorithms together in a single algorithm to estimate the set of parameters of our model which has been designed for the estimation of breakpoints (i.e., structural breaks). For a given set of parameter values \(\{\varvec{\theta }^{(s)}, \sigma ^{2(s)}, \rho ^{(s)}\}\) for sth MCMC iteration, the next iteration \(s + 1\) is generated as following:

1.
Update \(\sigma ^2\) using Gibbs sampling:

(a)
Calculate the sum of squares of error given \(\varvec{\theta }^{(s)}\) and \(\rho ^{(s)}\)
$$\begin{aligned} \ \text {SSE}_{\rho ^{(s)}}(\varvec{\theta }^{(s)}) = \left( {\mathbf {y}}  E({\mathbf {Y}}{\mathbf {x}}, \varvec{\theta }^{(s)})\right) ^T C_{\rho ^{(s)}}^{1} \left( {\mathbf {y}}  E({\mathbf {Y}}{\mathbf {x}}, \varvec{\theta }^{(s)})\right) . \end{aligned}$$ 
(b)
Generate a new value
$$\begin{aligned} \ \sigma ^{2(s+1)} \sim \text {InverseGamma}\left( \frac{\nu _0 + n}{2}, \frac{\nu _0 \sigma _0^2 + \text {SSE}_{\rho ^{(s)}}(\varvec{\theta }^{(s)})}{2}\right) , \end{aligned}$$where the posterior distribution is given by Eq. 7.

(a)

2.
Update \(\varvec{\theta }\) using Metropolis algorithm:

(a)
Given the current state of \(\varvec{\theta }^{(s)}\), propose \(\varvec{\theta }^*\) from a symmetric proposal distribution \(J_1(\varvec{\theta } \varvec{\theta }^{(s)})\)

(b)
Given the parameter values \(\varvec{\theta }^{(s)}\), \(\sigma ^{2(s+1)}\), \(\rho ^{(s)}\), and the proposal \(\varvec{\theta }^*\), calculate the acceptance ratio for the proposal
$$\begin{aligned} \ r_{\theta ^*} = \frac{P\left( {\mathbf {y}}{\mathbf {x}}, \varvec{\theta }^*, \sigma ^{2(s+1)}, \rho ^{(s)}\right) P\left( \varvec{\theta }^*\right) }{P\left( {\mathbf {y}}{\mathbf {x}}, \varvec{\theta }^{(s)}, \sigma ^{2(s+1)}, \rho ^{(s)}\right) P\left( \varvec{\theta }^{(s)}\right) }, \end{aligned}$$where the expression for the numerator and denominator is given in Eq. 8.

(c)
Accept or reject the proposal as following
$$\begin{aligned} \ \varvec{\theta }^{(s+1)} = \left\{ \begin{array}{ll} \varvec{\theta }^* &{}\quad \text {with probability} \ \min (r_{\theta ^*}, 1)\\ \varvec{\theta }^{(s)} &{}\quad \text {with probability} \ 1 \min (r_{\theta ^*}, 1). \end{array}\right. \end{aligned}$$

(a)

3.
Update \(\rho \) using Metropolis algorithm:

(a)
Given \(\rho ^{(s)}\), propose \(\rho ^*\) from a symmetric proposal distribution \(J_2(\rho ^*\rho ^{(s)})\)

(b)
Given the parameter values \(\varvec{\theta }^{(s+1)}\), \(\sigma ^{2(s+1)}\), \(\rho ^{(s)}\), and the proposal \(\rho ^*\), calculate the acceptance ratio for the proposal
$$\begin{aligned} \ r_{\rho ^*} = \frac{P\left( {\mathbf {y}}{\mathbf {x}}, \varvec{\theta }^{(s+1)}, \sigma ^{2(s+1)}, \rho ^*\right) P\left( \rho ^*\right) }{P\left( {\mathbf {y}}{\mathbf {x}}, \varvec{\theta }^{(s+1)}, \sigma ^{2(s+1)}, \rho ^{(s)}\right) P\left( \rho ^{(s)}\right) }, \end{aligned}$$where the expression for the numerator (and denominator) is given in Eq. 9.

(c)
Accept or reject the proposal as following
$$\begin{aligned} \ \rho ^{(s+1)} = \left\{ \begin{array}{ll} \rho ^* &{}\quad \text {with probability} \ \min (r_{\rho ^*}, 1)\\ \rho ^{(s)} &{}\quad \text {with probability} \ 1 \min (r_{\rho ^*}, 1). \end{array}\right. \end{aligned}$$

(a)
3.6 Proposal distributions

1.
The proposal distribution for \(\varvec{\theta }\) is considered as
$$\begin{aligned} \ J_1\left( \varvec{\theta }\varvec{\theta }^{(s)}\right) = \text {MVN}\left( \varvec{\theta }^{(s)}, \Sigma _{p}\right) , \end{aligned}$$where \(\Sigma _p\) is the proposal variancecovariance matrix of \(\varvec{\theta }\).

2.
We set the proposal distribution for \(\rho \) as
$$\begin{aligned} \ J_2\left( \rho  \rho ^{(s)}\right) = \text {Unif}\left( \rho ^{(s)}  \delta , \rho ^{(s)} + \delta \right) . \end{aligned}$$If \(\rho ^* < 0\), reassign it to be \(\rho ^*\). If \(\rho ^* > 1\), reassign it to be \(2\rho ^*\).
3.7 Prior specification
In order to specify the hyperparameters for the prior distribution of \(\sigma ^2\), we depend on the loess fits. We calculated the residual variance from the loess fit, and assign that as \(\sigma _0^2\) to be \(17{,}037.93^2\) and \(12{,}080.95^2\) for Chilliwack and Kamloops data, respectively. We further use a noninformative prior and assign \(\nu _0 = 1\) for both sets of data. This ensures minimal effects of our prior knowledge on the posterior model.
To specify the hyperparameters for the prior distribution of \(\varvec{\theta }\), we depended on the prior knowledge, the hypotheses, and the raw data. The hyperparameter vector \(\varvec{\mu }_0 = (\theta _{00}, \theta _{10}, \theta _{20}, \theta _{30}, \theta _{40}, \theta _{50})\) is specified as follows. On July 25, 2016, the provincial government of British Columbia introduced Bill 28, Miscellaneous Statutes (Housing Priority Initiatives) Amendment Act, 2016. The bill amended the Property Transfer Tax Act to include an additional 15% transfer tax on foreign entities buying property in Metro Vancouver. We hypothesize that this bill introduces threshold effects in the surrounding cities of the Lower Mainland of British Columbia and reaching as far as Kamloops. This hypothesis helps us specifying \(\theta _{40}\) to be 67 (as July 2016 is the 67th month counting from January 2011). On February 20, 2018, the government of British Columbia extended the foreign buyer property transfer tax to the following regions (i) Capital Regional District, (ii) Fraser Valley Regional District, (iii) Metro Vancouver Regional District, (iv) Regional District of Central Okanagan, and (v) Regional District of Nanaimo. We further hypothesize that the extension of the foreign buyer tax to these regions introduces threshold effects in the cities within these regions. This hypothesis helps us to specify \(\theta _{50}\) to be 86 (as February 2018 is the 86th month counting from January 2011). The fitted nonparametric loess model helped us to determine the other hyperparameters for Chilliwack, BC data. We used the fitted loess values at some index time points (e.g., the first, 67th, 86th and last months) to calculate the slopes reasonable in the three segments of the data. We then used the relationships between the parameters and slopes to produce the following vector of hyperparameters for Chilliwack, BC as \(\varvec{\mu }_0 = (275573.80, 1860.78, 3859.67, 3589.30, 67, 86)\). Following the same steps, we specify the hyperparameters for Kamloops, BC as \(\varvec{\mu }_0 = (308941.00, 462.53, 1456.33, 866.66, 67, 86)\).
To specify the hyperparameter \(\Sigma _0\), we use statistical knowledge and common sense. In every specification, we make the prior distribution flat and diffuse leading to large prior variance. We define the prior covariance matrix for Chilliwack first. We consider that the covariances between \(\theta _i\) and \(\theta _j\) are zero. The standard deviation of \(\theta _0\) is considered as 10,000. For the parameters \(\theta _1\), \(\theta _2\) and \(\theta _3\) (parameters specific to the slopes), we consider the standard deviation such as 1900, 3900 and 3600, respectively. Such specification ensures that each of the prior distributions of \(\theta _1\), \(\theta _2\) and \(\theta _3\) includes 0 well within the high probability region. For the threshold parameters \(\theta _4\) and \(\theta _5\), we consider prior distributions by assigning enough masses within 24month time spans. This gives us a prior covariance matrix for Chilliwack as
Similarly, the prior covariance matrix for Kamloops is considered as
Such specification of hyperparameters make the prior distributions for \((\theta _0, \theta _1, \theta _2, \theta _3)\) diffuse and flat leading to negligible effect on the posterior distributions. On the other hand, the priors variances for \(\theta _4\) and \(\theta _5\) (the breakpoints) make their priors informative. The reason for using informative prior is that [2] suggested to use informative prior instead of vague, flat or diffuse prior for the parameters whenever prior information is available.
3.8 Initial values of the parameters
In order to start the Metropolis and Gibbs sampling algorithm, we use the following initial values \(\{\varvec{\theta }^{(s)}, \sigma ^{2(s)}, \rho ^{(s)}\} = \{\varvec{\mu }_0, \sigma ^2_0, {\hat{\rho }}_1\}\), where \({\hat{\rho }}_1\) is the lag1 autocorrelation of the residuals obtained from the fitted loess. The lag1 autocorrelations of the loess residuals are calculated as \({\hat{\rho }}_1 = 0.30\) for Chilliwalk and \({\hat{\rho }}_1 = 0.12\) for Kamloops.
To determine the proposal covariance matrix \(\Sigma _p\), we ran the algorithm in two phases. In the first phase, we considered \(\Sigma _p = \Sigma _0/k\), and determined a reasonable value of k using trialanderror method. In the first phase, a total of 22,000 MCMC iterations were generated and the first 2000 iterations were used as burnin. Among the 20,000 MCMC iterations after burnin, we saved every 20th iteration for the \(\varvec{\theta }\) vector as part of thinning. The empirical covariance matrix of the thinned values for \(\varvec{\theta }\) was used as the proposal covariance matrix \(\Sigma _p\) for the second phase of the algorithm. In the second phase, the algorithm was run for a total of 120,000 iterations and the first 20,000 iterations were used as burnin. After throwing out the burnin iterations, in the second phase, every 100th iteration was saved as part of thinning. This provided us exactly 1000 values for each parameter after burnin and thinning.
3.9 MCMC diagnostics
The second phase of the algorithm gave us an acceptance rate of \(21.58\%\) and \(77.05\%\) for \(\varvec{\theta }\) and \(\rho \), respectively, for the data from Chilliwack, BC. Fig. 2 shows the autocorrelation functions for each of the \(\theta \)’s. As usual, the generated MCMC iterations are highly autocorrelated. This justifies our approach of thinning the generated MCMC iterations. After thinning, the effective sample sizes for \(\theta _0\), \(\theta _1\), \(\theta _2\), \(\theta _3\), \(\theta _4\), \(\theta _5\), \(\sigma \), and \(\rho \) are 1000, 1000, 1000, 1000, 1000, 1000, 1000, and 995, respectively. Fig. 3 shows the autocorrelation functions for each of the \(\theta \)’s for the data obtained from Kamloops, BC. As usual, the generated MCMC iterations are highly autocorrelated. After thinning, the effective sample sizes for \(\theta _0\), \(\theta _1\), \(\theta _2\), \(\theta _3\), \(\theta _4\), \(\theta _5\), \(\sigma \), and \(\rho \) are 1000, 1000, 1000, 479, 1000, 444, 1000, and 1000, respectively.
Figure 4 shows the trace plots of the generated regression coefficients after burnin and thinning for Chilliwack data. The plots show that the Markov Chains achieved stationarity. Similarly, the stationarity for the MCMC iterations of the coefficients were also checked for the Kamloops data (Fig. 5).
Note that the four parameters (\(\theta _0\), \(\theta _1\), \(\theta _2\) and \(\theta _4\)) in Fig. 3 decayed quickly, but not \(\theta _3\) and \(\theta _5\). This is because, for Kamloops data, only the first threshold is statistically significant. Precisely, there is no significant difference between the slopes in the second and third segments, demarcated by the threshold \(\theta _5\), represented by \(\theta _1 + \theta _2\) and \(\theta _1 + \theta _2 + \theta _3\) (details are reported in Fig. 6 and Table 2). From computational perspective, the estimates of \(\theta _3\) were scattered around 0 and showed greater autocorrelation. This indicates strong evidence towards insignificant second threshold effect in Kamloops, BC. The corresponding, traceplots in Fig. 5 show that the MCMC iterations mixed nicely and their distributions achieved stationarity. To our support, the ACFs of the six parameters for Kamloops data (figure not shown) after burnin and thinning showed negligible autocorrelation between the saved MCMC iterations. Finally, after burnin and thinning, the effective sample sizes for \(\theta _3\) and \(\theta _5\) became 479 and 444, respectively, which are reasonably large to effectively measure various statistical properties.
4 Results
For all the parameters of our model, the point estimates represent the median of the posterior distribution of the parameters calculated from the thinned MCMC samples. Moreover, the lower and upper limits of the \(95\%\) credible intervals are calculated using the 2.5th and 97.5th percentiles, respectively, of the posterior distribution.
Figure 6a shows the \(95\%\) credible intervals (equivalent to confidence intervals in classical statistics) for \(\theta _2\) and \(\theta _3\) for Chilliwack data. None of the credible interval of \(\theta _2\) and \(\theta _3\) includes 0 value inside, making the thresholds of \(\theta _4\) and \(\theta _5\) statistically significant. Table 1 shows the \(95\%\) credible intervals for the breakpoints \(\theta _4\) and \(\theta _5\). The breakpoints occurred in months of 54 (April 2015) and 87 (March 2018) with nonoverlapping intervals. Hence, the two breakpoints are significantly different from each other. Fig. 6b and bottom part of Table 1 show the \(95\%\) credible intervals of the slopes. In the first segment of the model (from January 2011 to April 2015), the slope \(\theta _1\) is 686.75 with \(95\%\) credible interval ranging from 288.17 to 1112.54: The average price increase per month is \(\$686.75\). In the second segment of the model (from April 2015 to March 2018), the slope \(\theta _1 + \theta _2\) is 5681.86 with \(95\%\) credible interval ranging from 4965.46 to 6857.23: The average price increase per month is \(\$5681.86\). In the third segment of the model (from March 2018 to July 2020), the slope \(\theta _1 + \theta _2 + \theta _3\) is 1281.52 with \(95\%\) credible interval ranging from 381.15 to 2232.62: The average price increase per month is \(\$1281.52\). The price increases in the first and third segments are not significantly different as the two credible intervals overlap each other.
Figure 6c shows the \(95\%\) credible intervals for \(\theta _2\) and \(\theta _3\) for the Kamloops data. The credible interval for \(\theta _2\) does not include zero inside, and, thus, the threshold effect \(\theta _4\) is statistically significant. On the other hand, the credible interval of \(\theta _3\) includes zero inside, and the threshold effect of \(\theta _5\) is statistically insignificant. Table 2 shows the \(95\%\) credible intervals for breakpoint \(\theta _4\) which occurred in month of 58 (August 2015) with \(95\%\) credible interval ranging from 50.17 to 65.82. Fig. 6d and bottom part of Table 2 show the \(95\%\) credible intervals for the slopes. In the first segment of the model (from January 2011 to August 2015), the slope \(\theta _1\) is 407.87 with \(95\%\) credible interval ranging from 161.37 to 623.11: The average price increase per month is \(\$407.87\). In the second segment of the model (from August 2015 to February 2018), the slope \(\theta _1 + \theta _2\) is 1991.66 with \(95\%\) credible interval ranging from 1449.35 to 2608.90: The average price increase per month is \(\$1991.66\). Although the effect is insignificant in the third segment of the model defined from February 2018 to July 2020, the slope \(\theta _1 + \theta _2 + \theta _3\) is 2093.71 with \(95\%\) credible interval ranging from 1500.72 to 2889.25: The average price increase per month is \(\$2093.71\). The price increases in the second and third segments are not significantly different from each other as the two credible intervals are highly overlapping.
Having seen the numbers and figures relating the regression coefficients, we produce the expected regression model (expected home price over linear trend in time) with \(95\%\) credible intervals. Fig. 7a shows the expected home price plotted against linear trend in time (in month) for Chilliwack data highlighting the breakpoints along the horizontal axis with \(95\%\) credible intervals. As noted before, the first and second breakpoints occurred in June 2015 and March 2018, respectively. After the second breakpoint, the slope in the third segment becomes similar to the slope in the first segment. Fig. 7b shows the expected home price plotted against linear trend in time (in month) for Kamloops data highlighting one breakpoint along the horizontal axis with \(95\%\) credible interval. The breakpoint occurred in August 2015 and the slope remained steep ever since.
It is interesting to note that the first breakpoint in Chilliwack occurred in June 2015 while the foreign buyer’s tax was introduced in Vancouver on August 2, 2016. It appears that the debate and discussions surrounding this policy change and the resulting expectations of the impact on the cost of buying homes in Vancouver led to the shift in demand for housing in other cities in BC even before the tax was finally introduced. The breakpoint in Kamloops came with a 2month time lag in August 2015 possibly for it being further away from Vancouver than Chilliwack.
To better understand the reasons for shifting the breakpoint in Chilliwack earlier than the date of the introduction of higher property transfer tax, we may need to consider the context for which the Government of British Columbia introduced the higher property transfer taxes to foreign buyers. Note that the home prices in Metro Vancouver had been increasing sharply in the years preceding 2016. When the home prices already became unbearable, some buyers might have shifted their interests of buying homes from Metro Vancouver to the nearby cities like Chilliwack. This shift of interest might have caused the rapid increase of home prices in Chilliwack earlier in 2016. Later, when the Government of BC introduced the higher property transfer taxes for foreign buyers it would have caused increased influx of buyers to the neighbouring city of Chilliwack, resulting in further rapid changes in home prices there. In that light, the early structural break in home prices in Chilliwack may therefore have been caused by these additional contributing factors along with the strong expectations of the tax to be introduced by the government.
Finally, we finish the results section by reporting the numbers regarding the lag1 autocorrelation \(\rho \) among the residuals. The median of the posterior realization of \(\rho \) is 0.274 with \(95\%\) credible interval ranging from 0.064 to 0.485 for Chilliwack data. For Kamloops data, the median of the posterior realization of \(\rho \) is 0.168 with \(95\%\) credible interval ranging from 0.024 to 0.361. The numbers show nonnegligible autocorrelation among successive residuals of the Bayesian piecewiselinear regression model.
Also, the median of the posterior standard deviation of residuals is 16,659.21 with \(95\%\) credible interval ranging from 14,449.39 to 19,674.93 for Chilliwack, BC. The median of the posterior standard deviation of residuals is 12,514.00 with \(95\%\) credible interval ranging from 10,933.89 to 14,422.36 for Kamloops, BC.
Note that the robustness of the results has been assessed by making the prior distributions of \(\varvec{\theta }\) further diffuse. Specifically, we multiplied the prior variances for \(\theta _0\), \(\theta _1\), \(\theta _2\) and \(\theta _3\) by \(2^2\), and for \(\theta _4\) and \(\theta _5\) by \(4^2\). These changes caused a few small changes in the results, here and there, but did not alter the inference. Therefore, we consider that the results obtained from our model are robust against changing priors from informative to noninformative.
5 Discussion
The Provincial Government of British Columbia introduced Bill 28, Miscellaneous Statutes (Housing Priority Initiatives) Amendment Act, 2016 on July 25, 2016. The introduction of an additional Property Transfer Tax on foreign entities buying property in Metro Vancouver caused threshold effects in housing prices in nearby cities such as Chilliwack, and as far as Kamloops. The threshold effect in Chilliwack happened relatively earlier than the threshold effect in Kamloops. This happened most likely because Chilliwack is situated within a closer proximity of the Metropolitan Vancouver than Kamloops which is 350 km farther. We iterate that the threshold effects in both cities occurred more or less at the same time as the two credible intervals (one to each city) highly overlapped each other.
Most importantly, on February 20, 2018, the Government of British Columbia extended the foreign buyer additional property transfer tax to the following regions: Capital Regional District, Fraser Valley Regional District, Metro Vancouver Regional District, Regional District of Central Okanagan, and Regional District of Nanaimo. These extended regions for additional property transfer tax to foreign entities include Chilliwack, but not Kamloops. As a result of the implementation of this policy Chilliwack experienced a second threshold in March 2018 with \(95\%\) credible interval extending from September 2017 to July 2018. This policy implementation from the local government of British Columbia did not cause any threshold effects via the changes in slopes in the Kamloops housing market.
The housing prices in Chilliwack show three linear trend segments demarcated by the two thresholds. The first threshold caused the housing prices to shoot up very quickly, whereas the second threshold caused a price correction by stabilizing the growth in housing prices back to normal as in the first segment. In contrast, there is only one threshold in Kamloops housing market due to the change in government policy back in July 2016. The housing market in Kamloops is sky rocketing since then with rapid growth in home prices quantifying \(513.33\%\) higher after threshold effect than the price growth before October 2015.
In a recent paper, Tomal and Ciborowski [42] developed a piecewise linear regression model with two breakpoints, where the model is defined through the conditional mean of the response variable given the predictor, and applied the model to two ecological datasets to detect environmental thresholds caused by human induced disturbances to aquatic habitat for fishes in the Great Lakes Coastal Wetland. They used classical method such as nonlinear least squares for parameter estimation and inference. Moreover, they developed a piecewise linear quantile regression model which is defined through the conditional quantile of the distribution of the response variable given the predictor. In this paper, we proposed the Bayesian counterpart of the twothresholds piecewise linear regression model, defined the priors, derived the posteriors, and presented the Metropolis and Gibbs sampling algorithm to generate data from the posterior. The proposed model is further extended to incorporate nonindependent correlated error terms suitable for time series data. The methods have been applied to the housing market data of two nearby cities in British Columbia, Canada to estimate the threshold effects resulting from the implementation and changes of government tax policies.
There are methods in Business and Economics, Lagrange Multiplier unit root test [9] and Bayesian model averaging [36] and GARCH model [3], which can detect thresholds in the relationships between variables. These models detect thresholds in the distribution of the time series variables, and are not very useful in terms of interpretation of the relationship. On the other hand, there are non linear autoregressive models [39, 40] and partial structural change model [1] for time series data that can estimate the slopes for the relationships between variables while testing for the presence of breakpoints. However, these latter models are developed using classical statistical method. In contrast, our proposed model is fully Bayesian in nature. Besides, our proposed model comes with the strength of estimating the thresholds while performing statistical tests to determine if the estimated breakpoints are significant or not. While estimating the breakpoints, our model estimates the relationships via slopes which are easy to interpret. Moreover, our model comes with capabilities to test if the estimated slopes in the different segments of the data are statistically significant or not. We have proved the ability of our model via two applications in housing price trend analysis. Last but not the least, the proposed model is equipped with abilities to estimate the appropriate amount of autocorrelation that is present in the data points defined through the autocorrelation of error terms.
In a recent article, Tomal et al. [43] developed a missing value imputation method for a Bayesian hierarchical model to measure the effect of COVID19 on students’ marks. In that model, the authors used conjugate prior distributions and, eventually, proposed Gibbs sampling to generate data and missing values from the posterior distributions. In this paper, we have used conjugate prior for the variance parameter \(\sigma ^2\), and nonconjugate priors for the regression coefficients \(\varvec{\theta }\) and correlation coefficient \(\rho \) for the error term. As a result, we have used Gibbs sampling for the variance parameter, and Metropolis algorithm for the regression coefficients and correlation coefficient for the error terms.
In order to initiate the algorithm, one needs to come up with a good proposal distribution to yield a reasonable acceptance rate. It is difficult to come up with a reasonable proposal variancecovariance matrix for the regression coefficients vector especially when the dimension of the matrix is large. Even if one comes with a reasonable positive definite variancecovariance matrix, a compact matrix may produce high acceptance rate and a sparse matrix may produce low acceptance rate. Note that both low and high acceptance rates generate highly autocorrelated MCMC iterations, which contain less amount of information in the generated samples. To overcome this problem, we have proposed a twostep method. In the first step, we run the algorithm for a small number of iterations and obtain a reasonable approximation for the proposal covariance matrix. The variancecovariance matrix for the regression coefficients is then used as the proposal distribution in the second step of the algorithm. The results show that this proposed method produces a nice balance in the acceptance rate and reasonable autocorrelation in the generated MCMC iterations. This method of dealing with the proposal distribution in higher dimension may become useful in many computational algorithms used in Bayesian statistical methods. Our twostep updating method for the proposal distribution is similar in nature to the adaptive Metropolis and Hastings algorithm proposed by [8, 21].
Note that the other two cities within the proximity of Vancouver, British Columbia are Victoria and Kelowna. These cities have experienced their own threshold effects from the introduction of property transfer taxes by the provincial government of British Columbia. Importantly, the patterns of threshold effects in these two cities are different than those observed in Chilliwack and Kamloops. The reasons for different patterns could be that the cities of Victoria and Kelowna are much different in size and business capacity than Chilliwack and Kamloops. Also, each of them houses a different sized university than the universities in Chilliwack and Kamloops. In summary, the differences of threshold patterns in these two cities make the proposed model not directly applicable to the data from Victoria and Kelowna. However, at this moment, the authors of this paper are working on developing models to detect threshold effects in Victoria and Kelowna as well. All of these results, when available and published, might help the readers to gain a better understanding of spatial effects of the introduction of property transfer taxes on housing prices in cities nearby Vancouver, BC.
In this paper, we considered an InverseGamma prior for \(\sigma ^2\) and a Multivariate Normal prior \(\varvec{\theta }\). These conjugate prior distributions made the posterior distributions simpler and enabled us generating MCMC samples using Gibbs sampling. As alternatives, one may wish to use other prior distributions as they fit instead of the provided priors. However, the use of alternative prior distributions may complicate the computational process especially when the posterior distributions are not in closed form. In such a situation, one may need to use Metropolis or MetropolisHastings algorithm instead of Gibbs sampling for \(\sigma ^2\) and \(\varvec{\theta }\). On the other hand, the applications of opensource MCMC softwares, such as JAGS [13], WinBUGS [29], or Stan [7] may appear handy to improve computational issues.
The proposed methodology of this paper might appear useful to model other macroeconomic variables such as gross domestic product, economic growth, inflation, and unemployment rates of a country to detect potential threshold effects of some events defined over time using Eq. 1 after careful exploratory analysis of the data using scatterplot such as in Fig. 7 and autocorrelation function plot as in Fig. 1. To understand the effects of financial crisis on international asset diversification in real estate market, via structural breaks in increased volatility during a high turbulent period, the readers are encouraged to read the article by Gerlach et al. [20]. Furthermore, to detect structural breaks using autoregressive model with exogeneous inputs (ARX) and GARCH model for the conditional variances, via volatility during a high turbulent period, we encourage readers to read the article by ThanThi et al. [41], which examined the impact of inflation and interest rates on housing price dynamics.
Notes
https://www2.gov.bc.ca/gov/content/taxes/propertytaxes/propertytransfertax/additionalpropertytransfertax (Accessed on March 7, 2021).
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The authors thank the Canadian Real Estate Association (CREA) for making the data publicly available. The authors also thank the Editor, and two anonymous reviewers for their insightful comments, which substantially strengthened the arguments presented here.
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Tomal, J.H., Rahman, H. A Bayesian piecewise linear model for the detection of breakpoints in housing prices. METRON 79, 361–381 (2021). https://doi.org/10.1007/s40300021002238
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DOI: https://doi.org/10.1007/s40300021002238