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Archive for February, 2016

p-values for programmers

February 24th, 2016 No comments

Data analysis always contains a degree of uncertainty, in statistics this uncertainty is often expressed through the concept of p-value and possible interpretations of the data tested using the ideas behind null hypothesis testing. What is the best way of explaining the concept of p-value and null hypothesis testing to software developers?

For the empirical software engineering workshops I have been running, p-values get both a broad brush description and a use case in the form of null hypothesis testing (expressed as code). The broad brush approach to p-values worked and the null hypothesis as-code approach flopped miserably.

The broad brush approach defines p-values as a measure of uncertainty and leaves it at that, no mention of what exactly is uncertain. I quickly move on to discuss the more important point, from the real-world point of view, of selecting a p-value. Choice of p-value is one side of a cost/benefit business decision; would you sit in a lecture theater that had a 1 in 20 chance of collapsing? It all depends on what benefits accrued from sitting there.

Developers understand code and expressing the operations around the use of null hypothesis testing, as code, will obviously make everything clear. Perhaps my attempt to fit everything on one slide concentrated things a bit too much, or perhaps being firehosed with stuff to learn had left people yearning for the lunch that was fast approaching. The following function, accompanied by me babbling away was met with many blank stares.

The following function is my stab at explaining the operation of the null hypothesis and the role played by p-values. I hope that things are a bit clearer for readers who have not had me teaching them R in 15 minutes, followed by having a general what you need to know about probability and statistics in 30 slides.

void null_hypothesis_test(void *result_data, float p_value)
{
// H set by reality, its value is only accessible by running the appropriate experiments
if ((probability_of_seeing_data_when_H_true(result_data) < p_value) ||
     !H)
   printf("Willing to assume that H is false\n");
else
   printf("H might be true\n");
}
 
null_hypothesis_test(run_experiment(), 0.05);

Workshop on survival and time series analysis in empirical SE

February 12th, 2016 No comments

In January the material in my book on Empirical software engineering using R had its first exposure to professional software developers at a one day workshop (there was a rerun last week; slides here). The sessions were both fully booked, but as often happens on half turned up, around 15 at each workshop. A couple of people turned up expecting to be taught R and found themselves in a software engineering workshop that assumed the attendees could learn R in 10 minutes (because professional developers are experienced enough to learn the basics of programming in any language in that amount of time).

The main feedback was that people wanted to see more code, something to give them a starting point for the hands-on sessions (I hate seeing reams and reams of code on slides and had gone too far in the minimalist direction).

The approach of minimizing what developers have to learn/remember, even if it means using more computer resources (e.g., always using glm), went down very well.

The probability & statistics material, in the session before lunch, had a mixed reception; this is partly because I have not come up with a clear message and I’m trying to get over several disparate ideas, i.e., sampling issues, life is complicated and p-values. People were a bit unhappy about the life is complicated, data is messy and you have to think about what you are doing message.

I think the regression model building material was a hit. People saw the potential in fitting curves to data and found it easy to do once the parameters to plot, lines, glm and predict were sorted out. There was lots of interesting discussion around interpreting the fitted models, with me continually hamming on the point that p-value selection was a business risk factor decision and what constituted a good model depended on what it was going to be used for.

Those attending wanted to learn more, which is great since the main aim was to show people what useful things could be done to motivate them to go off and learn more (relatively little may be known about empirical software engineering, but there is more than can be fitted into a one day workshop)

There is a part-2 workshop in March and the plan is to cover survival analysis, time series analysis and if there is time something else. It will be assumed that people have the skill level of those who attended the first workshop, e.g., can write basic R and fit a simple regression model.

Thanks to The Rise for sponsoring the venue for all three workshops.

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pycparser: a serious entry in the not-written-in-C C-parser category

February 8th, 2016 2 comments

Sometimes it feels like everybody and his dog has had a go at writing a C parser. On the whole these parsers are small subsets and the choice of subset seems to be driven by the developer’s taste, what they find easy to do and how much time they are willing to put into the project.

Some time ago a blog post discussing parsing C type declarations made me think that pycparser might be a cut above the usual learning experience projects and a quick look showed it was quite good. I recently tried it out again on the examples from my C book and it did a surprisingly good job of handling this rather weird set of edge cases (it failed to handle the code in 20 out of 957 files).

There is a type of person who insists that the C parser used by the C source analysis project they are working on be written in a ‘high-level’ language, i.e., they don’t want to use one of the perfectly adequate (and correct) parsers written in C/C++. I’m not sure whether this is because having to actually use C would expose the poor state of their knowledge of the language, language snobbery (its ok to analyse C source, but write it, heaven forbid) or they are members of a One True Language guild.

Up until now the parser of choice, for people not wanting to use C/C++, was CIL (a slightly more up to date version on Github); used by Coccinelle and many other tools.

If you really don’t want to parse C source using a parser written in C/C++, I think pycparser has now reached the stage where it is worth considering, along with CIL.

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