(An excerpt from The Certified Software Quality Engineer Handbook by Linda Westfall – soon to be published by ASQ Quality Press)
A Software Quality Engineer (SQE) needs to know when and how to use different sampling techniques in order to effectively use sampling during product and project management, audits, testing, and product acceptance. Section VI.C.1 of the ASQ Certified Software Quality Engineer (CSQE) Body of Knowledge (BOK) says that an SQE should be able to, “Define and distinguish between sampling methods (e.g., random, stratified, cluster) as used in auditing, testing, product acceptance, etc.”. [CSQE BOK]
When the set of all possible items in a population is very large it may be too costly or time consuming to do a comprehensive analysis of all of the items. For example, during an audit, there is just not enough time or resources to talk to every auditee, witness every process step or look at every quality record. If the customer base is large, it may be too costly to survey all the customers to determine their satisfaction level. Evaluating or estimating attributes or characteristics of the entire system, process, product or project through a representative sample can be more efficient while still providing the required information. To legitimately be able to use a sample to extrapolate the results to the whole population requires the use of one of four statistical sampling methods.