Re: Re: Re: Software Design Resourcesby BrowserUk (Pope)
|on Aug 22, 2003 at 08:42 UTC||Need Help??|
Sorry. I don't think that I made that bit very clear. The statistics I gave were for the coverage achieved by the teams of test case writers prior to the introduction of the Random Testcase Generator. That is to say, there where a bunch of coders charged with the task of sitting down and writing programs to exercise given subsets of the APIs. They used thier 'best judgement' to write the programs such that they covered the edge cases of each individual function an combination of functions.
The 15% was a purely mathematical count of the APIs exercised derived by simply grep'ing and counting them from the assembled test suit.
The 10% duplication meant that of the 15% that had actually been exercised, two thirds of them had been exercised in more than one testcase. For some parts of the API set this is enevitable. You can't do anything when testing a GUI API set without having called CreateWindow() for example, but this did not explain all the duplication.
Much of it came down to the fact that given any two programmers with similar experience, their best judgement, based on their prior experiences, will lead them to similar conclusions about what needs testing. Hence, they will tend towards testing similar things. Even though they are each assigned a particular set of API's to test, it's enevitable that there will be some overlap. Given a team of nearly 100 programmers from different backgrounds, you would think that their ranges of experience would lead to a fairly wide coverage, but it doesn't happen that way. They all tend to concentrate their efforts on similar clusters of "suspect" APIs. Worse, they all tend to assume that some APIs are not necessary to test, for similar reasons.
As for the 1% of possible bugs. The bit that I consider to be tantamount to voodoo, is the determination of the number of possible bugs. In order to state that "only 1 had been found", it is necessary to know how many were found and how many could have been found. How do you begin to determine how many there could be?
I fully understand the mechanism whereby it is possible to estimate how many bugs will be found on the basis of how many have been found, and projecting that forward, once the test cases are being produced randomly. This is fairly simple population sampling, standard deviation stuff. You only need to know that the sample is a truely random selection from the total population. You don't need to know the total population size.
But to conclude that 1% of possible bugs had been discovered by a set of testcases that the previous 2 statistics went soley to prove that their generation was anything but random, from the determanistic count of those that had been found, means that they had to have determined, or at least estimated to some degree of accuracy, the total possible bug count.
I have a good degree of faith in the guys doing the work, and I was treated to nearly four hours of explanation of the methodlogy involved. The program that produced that statistic ran on a top of the range S-370 quad processor system and consumed prodigious amounts of cpu-time. The datasets were not very large.
It involved a iterative process of refining a complex polynomial with an infinite number of terms, until it approximated the discovery rates and coverage that had been determined by counting. Once the polynimial in question had been refined until it closely approximated the real-world statistics it was developed to model, it was then iterated forward into the future to project to a point where no more bugs would be discovered. In real time this would have amounted to decades or maybe centuries. Once that point was reached, they then had the estimate of the number of bugs that could be discovered and it was this figure that was used to calculate the 1% figure.
Beleive me. This went way beyond graduate level statistics with which I was familar with at that time, though I have since forgotten much of it.
I'm going to stick to my guns and say that this was the deepest statistical voodoo that I have any wish to know about:)
Examine what is said, not who speaks."Efficiency is intelligent laziness." -David Dunham
"When I'm working on a problem, I never think about beauty. I think only how to solve the problem. But when I have finished, if the solution is not beautiful, I know it is wrong." -Richard Buckminster Fuller
If I understand your problem, I can solve it! Of course, the same can be said for you.