Derek Jones from The Shape of Code
What statistical techniques are of general usefulness for analyzing software engineering data?
The answer depends on the kinds of data likely to be encountered, in software engineering, and the questions likely to be asked.
When I started working on a book, aiming to cover all worthwhile publicly available software engineering data, I was hoping to refer readers to a book (or two) that they ought to read to learn the appropriate techniques. Kabacoff’s “R in Action” comes closest to the book I had in mind as a basic introduction, but there was nothing covering a wider range of topics; so I ended up writing something; I found Crawley’s “The R book”, to be the best book on the subject.
My answer to the kinds of data likely to be available was to work with all the software engineering data I could get obtain (around 600 data sets to date).
What questions should be asked about the data? My selection of questions was driven by whether the data was used in the software engineering half of the book, or the statistical analysis techniques half.
The software engineering material consists of the chapters: Introduction, Human cognitive characteristics, Cognitive capitalism, Ecosystems, Projects, Reliability and Source code. The data appeared in one of these chapters if it could be used to make (what I thought was) a practical point about the topic being discussed.
Data appeared in the statistical analysis techniques chapters, if it could be used to illustrate the technique under discussion.
What happened in practice was the software engineering material was worked on for a year or two, on realizing that bespoke statistical analysis material was needed the existing data was plundered to create the necessary chapters; after this was released, work switched back to the software engineering material (using unplundered and newly acquired data), and of course the earlier chapters plundered data from the yet to be worked on chapters.
This seems to have worked surprisingly well, at least from my perspective of keeping the production line going.
Now most if the data has been analyzed, it’s time to take a global overview and where necessary shuffle things around. I may find that everything is a complete mess; we shall see.
What techniques have I found to be useful?
The number 1, most useful data analysis technique is building a regression model. The one thing I have been consistently able to do, when analyzing other people’s data, is extract more information from it than they did (unless they also built a regression model); at times it has been embarrassing.
At number 2, is bootstrapping. Many widely used techniques only give accurate answers if the data has a normal/gaussian distribution and use of these techniques can involve a lot of arm waving involving claims about the data having a good-enough gaussian-like distribution. This arm waving was necessary before computers became available, because the practical manual techniques required a gaussian distribution. Now we have computers and techniques that don’t require any particular distribution can be used, and which in some cases are more powerful techniques than those designed for manual implementation.
Sitting here, I cannot think of a number 3; there might be one.
What techniques are not generally useful? The various tests containing some combination of the names Wilcoxon, Mann and Whitney are well past their sell-by date. Searching the source of the book I see these names still appear in one or two places; this is a hangover from the early versions from many years ago (when I was following the clueless herd) and will soon be gone.
I thought that extreme value theory might apply to some data, but have only found one data-set to which it might be applied (so not generally useful).
I spent a lot of time watching out for zero-inflated data (data containing more zero values than expected by the common probability distributions). I saw four/five papers containing plots of data that looked zero-inflated and emailed the authors asking for the data (who kindly sent it to me). None of the data turned out to be zero-inflated (I’m not sure what the authors thought about being asked for data that somebody thought was zero-inflated). This does not mean that software engineering data is not zero-inflated, only that it is not common.
My zero-inflated search was motivated by the occasional appearance of zero-truncated data (data with that does not contain zero values). Zero-truncated data occurs when counting starts at one, rather than zero (I have one data-set that is 0/1 truncated; the counting starts at 2).
I was surprised that time-series did not turn out to be widely useful.
Sometimes we are all clueless button pushers, so machine learning gets a few pages. Anybody who knows what they are doing builds regression models.
I will eventually get around to counting how many times each technique is used on the data I have (watch this blog, but don’t hold your breath).