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Polished statistical analysis chapters in evidence-based software engineering
I have completed the polishing/correcting/fiddling of the eight statistical analysis related chapters of my evidence-based software engineering book, and an updated draft pdf is now available (download here).
The material was in much better shape than I recalled, after abandoning it to the world 2-years ago, to work on the software engineering chapters.
Changes include moving more figures into the margin (which is responsible for a lot of the reduction in page count), fixing grammatical typos, removing place-holders for statistical techniques that are unlikely to be of general interest to software engineers, and mostly minor shuffling around (the only big change was moving a lot of material from the Experiments chapter to the Statistics chapter).
There is still some work to be done, in places (most notably the section on surveys).
What next? My collection of data waiting to be analysed has been piling up, so I will spend the next month reducing the backlog.
The six chapters covering the major areas of software engineering need to be polished and fleshed out, from their current bare-bones state. All being well, this time next year a beta release will be ready.
While working on the statistical material, I have been making monthly updates to the pdf+data available. If it makes sense to do this for the rest of the material, then it will happen. I’m not going to write a blog post every month; perhaps a post after what look like important milestones.
As always, if you know of any interesting software engineering data, please tell me.
Experimental Psychology by Robert S. Woodworth
I have just discovered “Experimental Psychology” by Robert S. Woodworth; first published in 1938, I have a reprinted in Great Britain copy from 1951. The Internet Archive has a copy of the 1954 revised edition; it’s a very useful pdf, but it does not have the atmospheric musty smell of an old book.
The Archives of Psychology was edited by Woodworth and contain reports of what look like ground breaking studies done in the 1930s.
The book is surprisingly modern, in that the topics covered are all of active interest today, in fields related to cognitive psychology. There are lots of experimental results (which always biases me towards really liking a book) and the coverage is extensive.
The history of cognitive psychology, as I understood it until this week, was early researchers asking questions, doing introspection and sometimes running experiments in the late 1800s and early 1900s (e.g., Wundt and Ebbinghaus), behaviorism dominants the field, behaviorism is eviscerated by Chomsky in the 1960s and cognitive psychology as we know it today takes off.
Now I know that lots of interesting and relevant experiments were being done in the 1920s and 1930s.
What is missing from this book? The most obvious omission is equations; lots of data points plotted on graph paper, but no attempt to fit an equation to anything, e.g., an exponential curve to the rate of learning.
A more subtle omission is the world view; digital computers had not been invented yet and Shannon’s information theory was almost 20 years in the future. Researchers tend to be heavily influenced by the tools they use and the zeitgeist. Computers as calculators and information processors could not be used as the basis for models of the human mind; they had not been invented yet.
What statistical techniques are useful for software engineering data?
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).
Source code chapter added to “Evidence-based software engineering using R”
The Source Code chapter of my evidence-based software engineering book has been added to the draft pdf (download here).
This chapter has suffered from coming last and there is still lots of work to be done. Almost all the source code related data has been plundered to fill up earlier chapters. Some data did not make the cut-off for release of the draft; a global review will probably result in some data migrating back to this chapter.
When talking to developers about the book I am constantly being asked ‘what is empirical software engineering?’ My explanation uses the phrase ‘evidence-based’, which everybody seems to immediately understand. It is counterproductive having a title that has to be explained, so I have changed the title to “Evidence-based Software Engineering using R”.
What is the purpose of a chapter discussing source code in a book on evidence-based software engineering? Source code is obviously an essential component of the topics discussed in the other chapters, but what is so particular to source code that it could not be said elsewhere? Having spent most of my professional life studying source code, first as a compiler writer and then involved with static analysis, am I just being driven by an attachment to the subject?
My view of source code is very different from most other developers: when developers talk about code, they spend most of the time talking about how they do things, when I talk about code I spend most of the time talking about how other developers do things (I’m a mongrel writer of code). Developers’ blinkered view of code prevents them seeing bigger pictures. I take a Gricean view of code and refrain from using meaningless marketing terms such as maintainability, readability and testability.
I have lots of source code data of interest to compiler writers (who are not the target audience) and I have lots of data related to static analysis (tool developers are not the audience). The target audience is professional software developers and hopefully what has been written is of interest to that readership.
I have been promised all sorts of data. Hopefully some of it will arrive. If somebody tells you they promised to send me data, please encourage them to take some time to sort out the data and send it.
As always, if you know of any interesting software engineering data, please tell me.
Finalizing the statistical analysis material in the second half of the book (released almost two years ago) next.
Reliability chapter added to “Empirical software engineering using R”
The Reliability chapter of my Empirical software engineering book has been added to the draft pdf (download here).
I have been working on this draft for four months and it still needs lots of work; time to move on and let it stew for a while. Part of the problem is lack of public data; cost and schedule overruns can be rather public (projects chapter), but reliability problems are easier to keep quiet.
Originally there was a chapter covering reliability and another one covering faults. As time passed, these merged into one. The material kept evaporating in front of my eyes (around a third of the initial draft, collected over the years, was deleted); I have already written about why most fault prediction research is a waste of time. If it had not been for Rome I would not have had much to write about.
Perhaps what will jump out at people most, is that I distinguish between mistakes in code and what I call a fault experience. A fault_experience=mistake_in_code + particular_input. Most fault researchers have been completely ignoring half of what goes into every fault experience, the input profile (if the user does not notice a fault, I do not consider it experienced) . It’s incredibly difficult to figure out anything about the input profile, so it has been quietly ignored (one of the reasons why research papers on reported faults are such a waste of time).
I’m also missing an ‘interesting’ figure on the opening page of the chapter. Suggestions welcome.
I have not said much about source code characteristics. There is a chapter covering source code, perhaps some of this material will migrate to reliability.
All sorts of interesting bits and pieces have been added to earlier chapters. Ecosystems keeps growing and in years to come somebody will write a multi-volume tomb on software ecosystems.
I have been promised all sorts of data. Hopefully some of it will arrive.
As always, if you know of any interesting software engineering data, please tell me.
Source code chapter next.
Data-set update to “Empirical software engineering using R”
The pile of papers, books and data-sets, relating to previously released draft chapters of my Empirical software engineering book, has been growing, and cluttering up my mind. I decided to have a clear-out.
A couple of things stood out.
There are around 25 data-sets that have been promised but not yet arrived. If you encounter anybody who mentions they promised to send me data, please encourage them to spend some time doing this. I don’t want to add a new category, promised but never delivered, to the list of email responses.
There has been an increase in data-sets not being used because I already have something better. This is a good sign, data quality is increasing. One consequence is that a growing number of ‘historical’ data-sets have fallen by the wayside. This is a good thing, most data-sets analysed in papers are very low quality and only used because nothing else was available.
One of my reasons for making draft releases was to prompt people to suggest data I had missed. This has not happened yet; come on people, suggest some data I don’t yet know about.
About a third of the pile got included in the latest draft, a third had been superseded by something better, and a third are still waiting for promised data.
Now, back to the reliability chapter.
Projects chapter added to “Empirical software engineering using R”
The Projects chapter of my Empirical software engineering book has been added to the draft pdf (download here).
This material turned out to be harder to bring together than I had expected.
Building software projects is a bit like making sausages in that you don’t want to know the details, or in this case those involved are not overly keen to reveal the data.
There are lots of papers on requirements, but remarkably little data (Soo Ling Lim’s work being the main exception).
There are lots of papers on effort prediction, but they tend to rehash the same data and the quality of research is poor (i.e., tweaking equations to get a better fit; no explanation of why the tweaks might have any connection to reality). I had not realised that Norden did all the heavy lifting on what is sometimes called the Putnam model; Putnam was essentially an evangelist. The Parr curve is a better model (sorry, no pdf), but lacked an evangelist.
Accurate estimates are unrealistic: lots of variation between different people and development groups, the client keeps changing the requirements and developer turnover is high.
I did turn up a few interesting data-sets and Rome came to the rescue in places.
I have been promised more data and am optimistic some will arrive.
As always, if you know of any interesting software engineering data, please tell me.
I’m looking to rerun the workshop on analyzing software engineering data. If anybody has a venue in central London, that holds 30 or so people+projector, and is willing to make it available at no charge for a series of free workshops over several Saturdays, please get in touch.
Reliability chapter next.
Books similar to my empirical software engineering book
I am sometimes asked which other books are similar to the Empirical Software Engineering book I am working on.
In spirit, the most similar book is “Software Project Dynamics” by Abdel-Hamid and Madnick, based on Abdel-Hamid’s PhD thesis. The thesis/book sets out to create an integrated model of software development projects, using system dynamics (the model can be ‘run’ to produce outputs from inputs, assuming the necessary software is available).
Building a model of the software development process requires figuring out the behavior of all the important factors and Abdel-Hamid does a thorough job of enumerating the important factors and tracking down the available empirical work (in the 1980s). The system dynamics model, written in Dynamo appears in an appendix (I have not been able to locate any current implementation).
In the 1980s I would have agreed with Abdel-Hamid that it was possible to build a reasonably accurate model of software development projects. Thirty years later, I have tracked down a lot more empirical work and know a more about how software projects work. All this has taught me is that I don’t know enough to be able to build a model of software development projects; but I still think it is possible, one day.
There have been other attempts to build models of major aspects of software development projects (all using system dynamics), including Madachy’s PhD and later book “Software Process Dynamics”, and Buettner’s PhD (no book, yet???).
There are other books that include some combination of the words empirical, software and engineering in their title. On the whole these are collections of edited papers, whose chapters are written by researchers promoting their latest work; there is even one that aims to teach students how to do empirical work.
Dag Sjøberg has done some interesting empirical work and is currently working on an empirical book, this should be worth a look.
“R in Action” by Kabacoff is the closest to the statistical material, but at a more general level. “The R Book” by Crawley is the R book I would recommended, but it is not at all like the material I have written.
Ecosystems chapter added to “Empirical software engineering using R”
The Ecosystems chapter of my Empirical software engineering book has been added to the draft pdf (download here).
I don’t seem to be able to get away from rewriting everything, despite working on the software engineering material for many years. Fortunately the sparsity of the data keeps me in check, but I keep finding new and interesting data (not a lot, but enough to slow me down).
There is still a lot of work to be done on the ecosystems chapter, not least integrating all the data I have been promised. The basic threads are there, they just need filling out (assuming the promised data sets arrive).
I did not get any time to integrate in the developer and economics data received since those draft chapters were released; there has been some minor reorganization.
As always, if you know of any interesting software engineering data, please tell me.
I’m looking to rerun the workshop on analyzing software engineering data. If anybody has a venue in central London, that holds 30 or so people+projector, and is willing to make it available at no charge for a series of free workshops over several Saturdays, please get in touch.
Projects chapter next.
Economics chapter added to “Empirical software engineering using R”
The Economics chapter of my Empirical software engineering book has been added to the draft pdf (download here).
This is a slim chapter, it might grow a bit, but I suspect not by a huge amount. Reasons include lots of interesting data being confidential and me not having spent a lot of time on this topic over the years (so my stash of accumulated data is tiny). Also, a significant chunk of the economics data I have is used to discuss issues in the Ecosystems and Projects chapters, perhaps some of this material will migrate back once these chapters are finalized.
You might argue that Economics is more important than Human cognitive characteristics and should have appeared before it (in chapter order). I would argue that hedonism by those involved in producing software is the important factor that pushes (financial) economics into second place (still waiting for data to argue my case in print).
Some of the cognitive characteristics data I have been waiting for arrived, and has been added to this chapter (some still to be added).
As always, if you know of any interesting software engineering data, please tell me.
I am after a front cover. A woodcut of alchemists concocting a potion appeals, perhaps with various software references discretely included, or astronomy related (the obvious candidate has already been used). The related modern stuff I have seen does not appeal. Suggestions welcome.
Ecosystems next.
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