It Will Never Work in Theory

A Multi-Site Joint Replication of a Design Patterns Experiment Using Moderator Variables to Generalize Across Contexts

Posted Sep 9, 2016 by Greg Wilson

| Replication | Design Patterns |

Jonathan L. Krein, Lutz Prechelt, Natalia Juristo, Aziz Nanthaamornphong, Jeffrey C. Carver, Sira Vegas, Charles D. Knutson, Kevin D. Seppi, and Dennis L. Eggett: "A Multi-Site Joint Replication of a Design Patterns Experiment Using Moderator Variables to Generalize Across Contexts". IEEE Trans. Software Engineering, 42(4), April 2016, 10.1109/TSE.2015.2488625.

Context. Several empirical studies have explored the benefits of software design patterns, but their collective results are highly inconsistent. Resolving the inconsistencies requires investigating moderators—i.e., variables that cause an effect to differ across contexts. Objectives. Replicate a design patterns experiment at multiple sites and identify sufficient moderators to generalize the results across prior studies. Methods. We perform a close replication of an experiment investigating the impact (in terms of time and quality) of design patterns (Decorator and Abstract Factory) on software maintenance. The experiment was replicated once previously, with divergent results. We execute our replication at four universities—spanning two continents and three countries—using a new method for performing distributed replications based on closely coordinated, small-scale instances ("joint replication"). We perform two analyses: 1) a post-hoc analysis of moderators, based on frequentist and Bayesian statistics; 2) an a priori analysis of the original hypotheses, based on frequentist statistics. Results. The main effect differs across the previous instances of the experiment and across the sites in our distributed replication. Our analysis of moderators (including developer experience and pattern knowledge) resolves the differences sufficiently to allow for cross-context (and cross-study) conclusions. The final conclusions represent 126 participants from five universities and 12 software companies, spanning two continents and at least four countries. Conclusions. The Decorator pattern is found to be preferable to a simpler solution during maintenance, as long as the developer has at least some prior knowledge of the pattern. For Abstract Factory, the simpler solution is found to be mostly equivalent to the pattern solution. Abstract Factory is shown to require a higher level of knowledge and/or experience than Decorator for the pattern to be beneficial.

This paper's conclusions about the efficacy of two particular design patterns are less important than its demonstration that claims about software design can be tested rigorously through carefully-designed studies. The experimental and statistical methods used would be immediately familiar to anyone working in public health, and the claims are carefully circumscribed. The end result is less rousing than the rhetoric found in many popular books, but I believe that's a sign of our field's increasing maturity.

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