Re: [SystemSafety] Qualifying SW as "proven in use" [Measuring Software]

From: Steve Tockey < >
Date: Tue, 25 Jun 2013 18:18:01 +0000

Derek,
We may end up having to respectfully agree to disagree on the cyclomatic complexity topic...

"I have some data whose writeup is not yet available in a good enough draft form to post to my blog. I only plan to write about this metric because it is widely cited and is long overdue for relegation to the history of good ideas that did not stand the scrutiny of empirical evidence."

I'm very interested to see what empirical evidence that you have, because that would be entirely counter to the experiences that I've had. I've worked on non-trivial code bases with both uncontrolled and controlled cyclomatic complexities, I would MUCH rather work on the controlled code bases.

"Correlation is not causation."

Absolutely. However, look at it this way: Clearly, it cannot be that an increase in defects is causing the increase in cyclomatic complexity. So, either something else is driving simultaneous increase in cyclomatic complexity and defect count, or cyclomatic complexity is (contributing into) increasing defect count. Based on my 36 years of dealing with non-trivial code bases, I'm willing to bet it's the cyclomatic complexity (contributing to) increasing defects.

"Cyclomatic complexity correlates well with lines of code, which in turn correlates well with number of faults." You are correct. However, I'm not looking at a straight correlation of lines of code to defects (e.g., 2x lines of code implies 2x defects). I'm looking at the correlation to *defect density*. Multiplying the lines of code by a factor of 10 may lead to a resulting increase in defects by essentially the same factor of 10. But I'm looking at holding the lines of code essentially constant and seeing more than an order of magnitude increase in *defect density* as cyclomatic complexity goes from <=5 to
>=15.

"Changing the function may increase the number of faults. Creating two functions where there was previously one will reduce an existing peak in the distribution of values, but will it result in less faults overall?"

Sorry, but I think you are oversimplifying here. Surely, pathological code changes can increase faults without changing cyclomatic complexity. Simply changing:

   if (a == b) {
     ...

to

   if (a != b) {
     ...

in previously working code introduces a fault without changing cyclomatic complexity. Remember that one should not be modifying the code in arbitrarily silly ways, one should be modifying the code to either remove known defects or to add new functionality. And the code modifications *should* be done in a meaningful, deliberate manner.

Besides, as a consulting buddy of mine has said many times, "Software complexity is not a number, it's a vector". There's no way (IMHO) one can capture all of the dimensions of code complexity into one single number. At best, one has to look at balancing a set of numbers. In my own coding work, I'm looking at maintaining a balance between 3 primary numbers:

   Cyclomatic complexity
   Depth of (decision) nesting
   Fan out
It's usually trivial to exchange cyclomatic complexity and/or depth of (decision) nesting for fan out and back. The trick is to keep them in an appropriate balance.

As well, I'm convinced that there are a whole slew of other complexity metrics that will prove to be relevant. I don't have the time to do the analysis myself, but I'd certainly contribute some of my time to an effort that did look into it.

Anyway, I do think you might be missing what I thought to be the far more important point I was trying to make--either that, or you agreed to it because there wasn't any comment on it. And that's the whole "paying attention to syntactic complexity metrics is necessary but not sufficient". One also needs to be paying attention to code quality & complexity at a semantic level.

Regards,

-----Original Message-----
From: Derek M Jones <derek_at_xxxxxx Organization: Knowledge Software, Ltd
Date: Monday, June 24, 2013 4:38 PM
To: "systemsafety_at_xxxxxx <systemsafety_at_xxxxxx Subject: Re: [SystemSafety] Qualifying SW as "proven in use" [Measuring Software]

All,

> Actually, getting the evidence isn't that tricky, it's just a lot of >work.

This is true of most things (+ getting the money to do the work).

> Essentially all one needs to do is to run a correlation analysis
> (correlation coefficient) between the proposed quality measure on the one
> hand, and defect tracking data on the other hand.

There is plenty of dirty data out there that needs to be cleaned up before it can be used:
http://shape-of-code.coding-guidelines.com/2013/06/02/data-cleaning-the-nex t-step-in-empirical-software-engineering/

> For example, the code quality measure "Cyclomatic Complexity" (reference:
> Tom McCabe, ©řA Complexity Measure©÷, IEEE Transactions on Software
> Engineering, December, 1976) was validated many years ago by simply

I am not aware of any study that validates this metric to a reasonable standard. There are a few studies that have used found a medium correlation in a small number of data points.

I have some data whose writeup is not yet available in a good enough draft form to post to my blog. I only plan to write about this metric because it is widely cited and is long overdue for relegation to the history of good ideas that did not stand the scrutiny of empirical evidence.

> finding a strong positive correlation between the cyclomatic complexity
>of
> functions and the number of defects that were logged against those same

Correlation is not causation.

Cyclomatic complexity correlates well with lines of code, which in turn correlates well with number of faults.

> functions (I.e., code in that function needed to be changed in order to > repair that defect).

Changing the function may increase the number of faults. Creating two functions where there was previously one will reduce an existing peak in the distribution of values, but will it result in less faults overall?

All this stuff with looking for outlier metric values is pure hand waving. Where is the evidence that the reworked code is better not worse?

> According to one study of 18 production applications, code in functions
> with cyclomatic complexity <=5 was about 45% of the total code base but
> this code was responsible for only 12% of the defects logged against the
> total code base. On the other hand, code in functions with cyclomatic
> complexity of >=15 was only 11% of the code base but this same code was
> responsible for 43% of the total defects. On a per-line-of-code basis,
> functions with cyclomatic complexity >=15 have more than an order of
> magnitude increase in defect density over functions measuring <=5.
>
> What I find interesting, personally, is that complexity metrics for
> object-oriented software have been around for about 20 years and yet
> nobody (to my knowledge) has done any correlation analysis at all (or, at
> a minimum they have not published their results).
>
> The other thing to remember is that such measures consider only the
> "syntax" (structure) of the code. I consider this to be *necessary* for
> code quality, but far from *sufficient*. One also needs to consider the
> "semantics" (meaning) of that same code. For example, to what extent is
> the code based on reasonable abstractions? To what extent does the code
> exhibit good encapsulation? What are the cohesion and coupling of the
> code? Has the code used "design-to-invariants / design-forchange"? One
>can
> have code that's perfectly structured in a syntactic sense and yet it's
> garbage from the semantic perspective. Unfortunately, there isn't a way
> (that I'm aware of, anyway) to do the necessary semantic analysis in an
> automated fashion. Some other competent software professionals need to
> look at the code and assess it from the semantic perspective.
>
> So while I applaud efforts like SQALE and others like it, one needs to be
> careful that it's only a part of the whole story. More work--a lot
> more--needs to be done before someone can reasonably say that some
> particular code is "high quality".
>
>
> Regards,
>
> -- steve
>
>
>
>
>
> -----Original Message-----
> From: Peter Bishop <pgb_at_xxxxxx
> Date: Friday, June 21, 2013 6:04 AM
> To: "systemsafety_at_xxxxxx
> <systemsafety_at_xxxxxx
> Subject: Re: [SystemSafety] Qualifying SW as "proven
> in	use"	[Measuring	Software]
>
> I agree with Derek
>
> Code quality is only a means to an end
> We need evidence to to show  the means actually helps to achieve the
>ends.
>
> Getting this evidence is pretty tricky, as parallel developments for the
> same project won't happen.
> But you might be able to infer something on average over multiple
>projects.
>
> Derek M Jones wrote:
>> Thierry,
>>
>>> To answer your questions:
>>> 1ˇĆ) Yes, there is some objective evidence that there is a correlation
>>> between a low SQALE index and quality code.
>>
>> How is the quality of code measured?
>>
>> Below you say that SQALE DEFINES what is "good quality" code.
>> In this case it is to be expected that a strong correlation will exist
>> between a low SQALE index and its own definition of quality.
>>
>>> For example ITRIS has conducted a study where the "good quality" code
>>> is statistically linked to a lower SQALE index, for industrial
>>> software actually used in operations.
>>
>> Again how is quality measured?
>>
>>> No, there is not enough evidence, we wish there would be more people
>>> working on getting the evidence.
>>
>> Is there any evidence apart from SQALE correlating with its own
>> measures?
>>
>> This is a general problem, lots of researchers create their own
>> definition of quality and don't show a causal connection to external
>> attributes such as faults or subsequent costs.
>>
>> Without running parallel development efforts that
>> follow/don't follow the guidelines it is difficult to see how
>> reliable data can be obtained.
>>
>

-- 
Derek M. Jones                  tel: +44 (0) 1252 520 667
Knowledge Software Ltd          blog:shape-of-code.coding-guidelines.com
Software analysis               http://www.knosof.co.uk

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systemsafety_at_xxxxxx Received on Tue Jun 25 2013 - 20:18:18 CEST

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