Keywords

1 Introduction

In designing industrial products, designers are trying to enhance the performance of the product as much as they can, under some balance with production cost. And of course, the reason to build-in high functionalities in the products is to fulfill customer requirements and enhance product attractiveness. For smartphones, it seems that the overall product performance has increased steadily over the years [1]. It means that the designers have focused and the consumers have required such increase of the specifications.

KANO model [2] made an important suggestion on this tendency. The model said product qualities are not always linear to customer satisfaction. The model explains that there are 3 major types of quality attributes and by focusing on the so-called “delighters,” customer satisfaction levels can be increased efficiently. But even for low-cost products, so-called “must-bes” qualities should not be eliminated. Some previous studies [3,4,5,6] have focused in applying the concept to environmentally benign design. By focusing on delighters, it might be possible to increase product attractiveness without enhancing product performance greatly that requires extra environmental burden.

However, it is not always possible to find delighters in real products, especially for matured products/markets, since people soon start to think they are must-bes, once it is build-in commercial products. The author surveyed weights of design specifications and estimated eco-efficiencies of the design specifications in a previous study [7], and explained that it is possible to enhance product eco-efficiency by focusing some specifications. The author notified customer’s voice such as “this specification is important, but I am satisfied at the current level.” This was an important suggestion to think that must-bes can be found and the bending points of the curves can be specified. Since the enhancement of product performances require certain amounts of environmental burdens, it is not eco-efficient to increase the performance beyond these points. Thus, the objective of this study is to clarify the design specifications of the case study product are delighters, one-dimensional or must-bes, and whether there is a bending point in each specification, by a customer survey. Through this effort, the study can clarify the optimum levels of design specifications, in the context of eco-design.

2 Why KANO Model

The simplified KANO model in three different quality attributes can be expressed as Fig. 1 [8]. The figure clearly shows that if we can find the bending point of the mut-be curve, it will greatly contribute in eco-efficient design. If there is no drastic change of manufacturing technologies and the environmental burden to increase the design specification level is proportional to the increase amount like memory capacity, eco-efficiencies of design specifications defined by Eq. (1) will peak at the bending point of the must-be. At the same time, it will be the same for one-dimensional and will continuously increase for delighters. But even for a one-dimensional, a bending point can be found. Finding the bending points will be helpful to extract suggestions in designing eco-efficient products. That was the motivation of the study and the reason to carry out a customer survey to know customer’s preference on design specifications.

Fig. 1.
figure 1

Simplified KANO model [7]

Full size image
$$Eco{\text {-}}efficiency=\frac{Increase\ of\ the\ customer\ satisfaction\ level}{Increase\ of\ the\ environmental\ burden} [kg-CO_{2}-1]$$
(1)

3 Internet Survey of Customer Satisfaction

3.1 Target Product Specifications

In choosing the case study example as a smartphone, some design specifications to investigate customer preference were selected as below. Four different specification levels were set for each specification.

  • Battery capacity

  • Storage capacity

  • Display size

  • Variations of smartphone case

  • Price

The first three specifications are the top three functions listed to be important functions for smartphones in the preliminary survey. “Variations of smartphone case” was found to be relatively important among the three features related to outlook of the product, those were color, texture and variations of smartphone case, investigated in the previous study. The last one, price, was designated because it is helpful to know the willingness to pay of other specification by comparing to the preference to price. It is also possible to check the accuracy of survey by calculating the customer preference for the different levels of price, since the reactions to different prices are thought to be linear to the price itself. This is the background of the pricing system which can be seen in the reverse linear relationship between price and demand in basic economics theory.

3.2 Outline of the Customer Survey

Table 1 shows the four different specification levels of five design specifications of smartphones. The four specification levels were determined based on the actual values in the current smartphone market. The customer survey was designed based on conjoint analysis method [9]. Assuming that there are no interactions between the five specifications, L16 orthogonal array which can be used for five attributes and four levels was used to design the conjoint cards. Each card was evaluated with full score of 10. The survey was made to 230 respondents via internet.

Table 1. Four levels of five design specifications
Full size table

3.3 Results of the Survey

The result of the customer survey shown in the previous section for five design specifications are indicated in Fig. 2 to Fig. 6 below.

Fig. 2.
figure 2

Relation between storage capacity and customer satisfaction

Full size image
Fig. 3.
figure 3

Relation between battery capacity and customer satisfaction

Full size image
Fig. 4.
figure 4

Relation between display size and customer satisfaction

Full size image
Fig. 5.
figure 5

Relation between variations of smartphone cases and customer satisfaction

Full size image
Fig. 6.
figure 6

Relation between price and customer satisfaction

Full size image

4 Estimation of Eco-efficiencies

4.1 Estimation of Environmental Burden

Since the customer satisfaction levels corresponding to the different specification levels were estimated through the customer survey, the numerator of eco-efficiency equation has been identified. In order to identify the denominator, environmental burden to increase design specifications were investigated and extrapolated through previous studies [10, 11]. Table 2 shows the environmental burden increase quantified by CO2 emission corresponding to the increase of three design specifications except for variations of case and price.

As for the increase of variations of smartphone case, since there are molding types and one-by-one manufacturing types, increase of CO2 emission per unit is not linear to the increase to the variations, since some of the variations of smartphone cases are made by molding and some are one by one manufactured. Table 3 shows the calculated values considering this. The value was calculated referring plastic parts fabrication of automobile [12]. As it was shown in Fig. 6 in the previous section, customer satisfaction increase corresponding to the price decrease is almost linear. So, it seems that the approach is reasonable. Eco-efficiencies of four design specifications were calculated.

Table 2. Increase of environmental burden due to performance enhancement
Full size table
Table 3. Increase of environmental burden due to variations increase of case
Full size table

4.2 Eco-efficiency Calculation

By using the data used in Figs. 2, 3, 4, 5 and 6 and the data shown in Table 2 and 3, it is possible to calculate eco-efficiency of performance enhancement as Tables 4, 5, 6 and 7. Figure 7 is the visual expression of these results. The figure shows that storage capacity has smaller eco-efficiency than others and some specifications have smaller speak value at the second level performances.

Table 4. Eco-efficiency of storage capacity increase
Full size table
Table 5. Eco-efficiency of battery capacity increase
Full size table
Table 6. Eco-efficiency of display size increase
Full size table
Table 7. Eco-efficiency of case variation increase
Full size table
Fig. 7.
figure 7

Deviation of eco-efficiencies

Full size image

5 Discussion

Considering the survey results shown in Figs. 2, 3, 4, 5 and 6, no delighter was found in the target design specifications of a smartphone. Among the four specifications except price, storage capacity seems like one-dimensional, while battery capacity and display size look like must-bes. Unlike the motivation of the study in the beginning, significance of finding bending points of the must-be curves was clarified through the study. By finding the points, it will be possible to designate the proper design specifications that are the critical points to stop the performance enhancement beyond there.

For the design specifications in the case study, it is not eco-efficient to focus on increase of storage capacity anymore, since the eco-efficiency is relatively low at all the levels. As it is shown in Fig. 7, contrarily for battery capacity and display size, since the eco-efficiency of performance enhancement is not that low, it is helpful to focus on these specifications. However, there are critical points where the performance should be stopped to increase at the point. For battery capacity, eco-efficiency is rather high till 4,000mAh. Thus, the suitable value of battery capacity can be 4,000mAh. As for the display size, since the eco-efficiency drastically decreases and is almost 0 for the increase from 5.8 inches to 6.8 inches, the optimum display size can be 5.8 inches.

This strategy to limit the product performance at a certain level is a paradigm shift for modern product design that has been aiming to enhance performance as much as possible. Although the actual values might change due to technological levels and market situations, there can be critical points of product performances that the designers should avoid to enhance them beyond the points. For a competitive and matured product, it is likely to have must-bes. Because, if someone find a delighter, it would be caught up by the competitor. Therefore, it is likely to have bending points in the performance-customer satisfaction curves.

6 Summary

The paper proposed a procedure to know the practical behaviors of product design specifications based on KANO model and carried out a customer survey to know that. Throughout the study, although no delighter has been found for the matured product like smartphones, the significance of must-bes, especially, finding bending points of the curves was explained in the context of eco-efficient design. Then, the paper showed a method to estimate the eco-efficiencies of specific designs, by dividing the increase of customer satisfaction levels by the increase of environmental burden to enhance product performance. Using this procedure, the paper indicated some design specifications should be focused for ecodesign of the case study. Future work is to increase the reliability of the customer survey and gather more design specifications and product examples to clarify whether such optimum design specifications can be found always.