Overly strict screening can sharply lengthen recruitment

What it is

Overrestrictive eligibility criteria turn characteristics unrelated to the question, or manageable in analysis, into hard gates. Eligibility, time, and cost can deteriorate nonlinearly because candidates must satisfy every condition. Strictness is not accuracy; a criterion is warranted when it controls a real confound, protects safety, or defines the target population.

Why it happens

Overall eligibility equals the product of each stage's conditional pass rate given prior gates; it is not the product of marginal probabilities for correlated criteria. Rare combinations are also difficult for existing channels to cover. Restriction can create an artificial homogeneous sample whose findings apply only to a narrow conjunction of device, frequency, occupation, and product history, trading away realistic heterogeneity for easier interpretation.

Studying it

Classify each criterion as safety/ethics, construct definition, quota, measurable covariate, or preference. Use historical recruitment or a pilot screener to estimate marginal and joint incidence and simulate how removal affects time and inference. Measure variables inside the study for stratification when exclusion is unnecessary. If relaxation changes the target question, separate studies rather than silently broadening one.

Where it stops holding

Clinical, safety-critical, and specialist tasks may require strict capability and risk criteria. Low incidence may indicate a poor channel rather than excessive screening. Self-reported experience is noisy near fine thresholds and adds boundary misclassification. Faster recruitment cannot override protections for children, vulnerable populations, or sensitive research.

Related

• Same group: Q1.11.1 Quotas cover target segments · Q1.11.2 Existing-user databases exclude prospects · Q1.11.3 No-shows require controlled standby recruitment
• Adjacent: Q1.05 Participant recruitment · Q1.16 Time and budget constraints
• Search terms: eligibility criteria · screening incidence · recruitment feasibility