CSQE cheat sheet

Software Quality Engineering

• Quality cost model (Crosby/PAF): prevention + appraisal = cost of conformance; internal failure + external failure = cost of nonconformance. External failure is typically the most expensive category because it includes warranty, liability, and lost goodwill.

• Cost of quality (COQ) ratio = total quality costs / total revenue (or total development cost). A falling COQ with stable defect escape rates indicates prevention is displacing failure costs.

• Defect detection effectiveness (DRE) = defects found before release / (defects found before release + defects found after release). Track by phase to locate weak verification stages.

• Defect removal efficiency (DRE per phase) = defects removed in a phase / defects present when the phase began (including those found later that existed at phase entry).

• Defect density = defects / size unit (KLOC, function points, or story points). Compare like-for-like size measures only; do not mix LOC and FP densities.

• Reliability growth models: exponential (Goel-Okumoto, Musa) assumes failure rate decreases exponentially per fault removed; S-shaped (Yamada) accounts for learning effects. Rayleigh/Weibull curves model cumulative defect arrival.

• MTBF = 1 / failure rate (lambda). If lambda is per hour, MTBF is in hours. Availability = MTBF / (MTBF + MTTR).

• Reviews vs. testing: inspections find requirement/design defects earlier and cheaper; a Fagan inspection has planning, overview, preparation, inspection meeting, rework, and follow-up stages, each with entry/exit criteria.

• Root cause analysis tools for defects: 5 Whys, fishbone (Ishikawa), Pareto ranking of defect categories, and fault tree analysis for safety-relevant failures. Pareto: ~80% of defects concentrate in ~20% of modules.