Posts Tagged ‘bayesian’

Will incorrect answers be biased towards one arm of an if-statement?

February 10th, 2013 No comments

Sometimes it is possible to deduce which arm of a nested if-statement will be executed by looking at the form of the conditional expression in the outer if-statement, as in:

if ((L < M) && (M < H))
   if (L < H)
      ; // Execution always end up here
      ; // dead code

but not in:

if ((L > M) && (M < H))
   if (L < H)
      ; // Could end up here
      ; // or here

I ran an experiment at the 2012 ACCU conference where subjects saw nested if-statements like those above and had to specify which arm of the nested if-statement would be executed.

Sometimes subjects gave an answer specifying one arm when in fact both arms are possible. Now dear reader, do you think these incorrect answers will specify the then arm 50% of the time and the else arm 50% or do you think that that incorrect answers will more often specify one particular arm?

Of course I should have thought about this before I started to analyse the data, but this question is unrelated to the subject of the experiment and has only just cropped up because of the unexpectedly high percentage of this kind of incorrect answer. I had an idea what the answer would be but did not stop and think about relative percentages, rushing off to write a few lines of code to print the actual totals, so now my mind is polluted by knowing the answer (well at least for one group of subjects in one experiment).

Why does this “one arm preference” issue matter? The Bayesians out there will insist that the expected distribution (the prior in Bayesian terminology) of incorrectly chosen arms be factored in to the calculation of the probability of getting the numbers seen in the results. The paper Belgian euro coins: 140 heads in 250 tosses – suspicious? gives a succinct summary of the possibilities.

So I have decided to appeal to my experienced readership, yes YOU!

For those questions where the actual execution cannot be predicted in advance, from knowledge of the relative values of variables appearing in the outer if-statement, when an incorrect answer is given should the analysis assume:

  • a 50/50 split of incorrect answers between each arm, or
  • subjects are more likely to pick one arm; please specify a percentage breakdown between arms.

No pressure, but the submission deadline is very late tomorrow.

The results from the whole experiment will get written up here in future posts.

Update (three days later): Nobody was willing to stick their head above the parapet :-(

There were 69 correct answers and 16 incorrect answers to questions whose answer was “both arm”. Ten of those incorrect answers specified the ‘then’ arm and 6 the ‘else'; my gut feeling was that there should be even more ‘then’ answers. If there was no “first arm” there is an equal probability of a subject’s incorrect answer appearing in either arm; in this case the probability of a 10/6 split is 12% (so my gut feeling was just hunger pangs after all).