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The Bootstrap Discovery Behaviour (BDB): a new outlook on usability evaluation

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Abstract

The value of λ is one of the main issues debated in international usability studies. The debate is centred on the deficiencies of the mathematical return on investment model (ROI model) of Nielsen and Landauer (1993). The ROI model is discussed in order to identify the base of another model that, respecting Nielsen and Landauer’s one, tries to consider a large number of variables for the estimation of the number of evaluators needed for an interface. Using the bootstrap model (Efron 1979), we can take into account: (a) the interface properties, as the properties at zero condition of evaluation and (b) the probability that the population discovery behaviour is represented by all the possible discovery behaviours of a sample. Our alternative model, named Bootstrap Discovery Behaviour (BDB), provides an alternative estimation of the number of experts and users needed for a usability evaluation. Two experimental groups of users and experts are involved in the evaluation of a website (http://www.serviziocivile.it). Applying the BDB model to the problems identified by the two groups, we found that 13 experts and 20 users are needed to identify 80% of usability problems, instead of 6 experts and 7 users required according to the estimation of the discovery likelihood provided by the ROI model. The consequence of the difference between the results of those models is that in following the BDB the costs of usability evaluation increase, although this is justified considering that the results obtained have the best probability of representing the entire population of experts and users.

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Notes

  1. Actually, only Nielsen et al. (1993) used λ, instead of p (Lewis 1994; 2006; Virzi 1992; Wright and Monk 1991; Schmettow 2008) in the formula 1, partly because they derived their formula from the “Poisson process” (see Nielsen and Landauer 1993). Many authors (Lewis 1994; 2006; Virzi 1992; Wright and Monk 1991; Schmettow 2008) use the formula (1) written as: P = 1 – (1 – p) n, where “P” is the total number of problems in the interface, “p” the probability of finding the average usability problem when running a single average subject test and “n” is the number of participants.

  2. In the review phase of this work, a reviewer claimed that “The authors should do a Monte Carlo resampling exercise to assess the extent to which randomly selected sets of 6 experts (for the CW data) and 7 users (for the TA data) find or fail to find at least 80% of the problems discovered by the full samples,” since, according to the reviewer’s opinion, “The authors simply state the different sample size estimates and appear to assume that the BDB are correct without further, evaluation or any tests of significance”. In accordance with the reviewer’s suggestions, we have added this section.

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Authors and Affiliations

  1. ECoNA—Interuniversity Centre for Research on Cognitive Processing in Natural and Artificial Systems, Sapienza University of Rome, Rome, Italy

    Simone Borsci, Alessandro Londei & Stefano Federici

  2. Department of Human and Education Sciences, University of Perugia, Perugia, Italy

    Stefano Federici

Authors
  1. Simone Borsci
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  2. Alessandro Londei
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  3. Stefano Federici
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Correspondence to Simone Borsci.

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Appendix: User scenarios

Appendix: User scenarios

  1. 1.

    A friend of yours is enrolled on a 1-year activity in social service. You are interested in finding more information about social service activities and acquiring information in order to apply for a one-year job. Go to the website http://www.serviziocivile.it/, find that information and download the documents for the job application.

  2. 2.

    A friend of yours, who lives in Rome, has some internet connection problems, so he or she telephones you for assistance. In fact, he or she is interested in social service work, but he or she does not know where the office is and when it is open in order to present his curriculum vitae. Go to the website http://www.serviziocivile.it/ in order to find that information for him or her.

  3. 3.

    You are interested in social service activities, so you go to the website http://www.serviziocivile.it/ in order to see whether this website offers a newsletter service, even though you are not enrolled on the social service activities. If the newsletter service requires you log in, sign up to the newsletter.

  4. 4.

    A friend of yours is working on a 1-year social service project in the Republic of the Philippines. You are interested in applying for a job on this project. Go to the website http://www.serviziocivile.it/ in order to find information about the project and whether is possible to obtain a job.

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Borsci, S., Londei, A. & Federici, S. The Bootstrap Discovery Behaviour (BDB): a new outlook on usability evaluation. Cogn Process 12, 23–31 (2011). https://doi.org/10.1007/s10339-010-0376-6

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