Overview
Sampling failures are the most common source of research invalidity: a study with excellent design and analysis produces findings that cannot be generalized beyond the sample if the sampling method systematically excludes part of the target population. Convenience sampling (using available participants) is appropriate for exploratory or pilot studies where generalizability isn't claimed, but it's regularly used for studies where population generalizability is explicitly claimed — producing findings that cannot support those claims.
The Sampling Strategy Framework matches sampling method to the research question's generalizability requirements, calculates required sample size with explicit statistical criteria, and documents what population the sample does and does not represent.