Exhaustive Testing


 The following is  a response i sent to Kit who commented on my blog on ‘Insufficient Testing’ ….

Thanks for your comment. It’s almost a catch 22 situation. One of the principles of testing (according to ISTQB) is that Exhaustive Testing is impossible – I agree but the question is how much do you test and when do you know enough is enough?

For a complex system my thoughts would center around risk and priorities as your starting point. The approach or method used would ultimately rest on what level of auditability you must provide to the Business (they ultimately make the decision to go or no go.) Personally I would still use Exploratory Testing (if I was ‘allowed’ to) because in my experience I would be more likely to find something of value more often than through scripts.

However, in saying that, if the test team is involved right at the beginning of the project through walkthroughs, reviews or inspections (or any other type of review)than clarification and understanding will no doubt increase amongst the testing team with regards to the system.

After doing a Wikipedia search on Dr. Deming, one of his quotes is quite applicable to software testing… “Acceptable Defects: Rather than waste efforts on zero-defect goals, Dr. Deming stressed the importance of establishing a level of variation, or anomalies, acceptable to the recipient (or customer) in the next phase of a process. Often, some defects are quite acceptable, and efforts to remove all defects would be an excessive waste of time and money.” It is known that major commercial software often ships with known (and unknown) defects – MS Windows, Firefox v2.0 etc – its is reasonable then for the business to decide how much of the ‘risk’ they wish to carry. Testers should provide the necessary information to enable business to make that decision (good or bad).

At one New Zealand bank that I worked in, the test team I became involved with tried hard to exhaustively tested everything in a very complex application. The upshot was that one release took almost 12 months to ‘complete’ testing (there were other factors involved – personnel, political and management)BUT I guarantee that they could not say that that application was bug free. So I guess that leads to the second question – how much is enough?

James Bach says “When I exhausted the concerns of my internal critic (and external critics I asked to review my work), I decided it was good enough” (refer http://www.satisfice.com/articles/how_much.shtml).

NASA’s software safety standard (http://satc.gsfc.nasa.gov/assure/nss8719_13.html) NASA-STD-8719.13A September 15, 1997 – Section 3.4.5 says “The test results shall be analyzed to verify that all safety requirements have been satisfied. The analysis shall also verify that all identified hazards have been eliminated or controlled to an acceptable level of risk. The results of the test safety analysis shall be provided to the ongoing system safety analysis activity.” What then is an acceptable level of risk and acceptable to whom? Risk is then defined in this document as “…As it applies to safety, exposure to the chance of injury or loss. It is a function of the possible frequency of occurrence of the undesired event, of the potential severity of resulting consequences, and of the uncertainties associated with the frequency and severity.” Also in the document under section 1.4 Tailoring it says “….The tailoring effort shall include definition of the acceptable level of risk, which software is to be considered safety-critical, and whether the level of safety risk associated with the software requires formal safety certification.” Therefore at the end of the day , it’s a business decision taken within context of the business. As testers, we can test complexity within the context of the project and report back our findings – it is then up to those charged with making the ‘big’ decisions, to make them – or not!


Author: bjosman

Principal Consultant at OsmanIT brian.osman@osmanit.com

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