Why combine selection methods? Incremental validity and the cost of the interview-only hire
No single method predicts performance well enough on its own, but methods that tap different things add incremental validity — a structured interview plus a cognitive or work-sample measure pushed composite validity above .60 in the classic data — which is why hiring on one unstructured interview is the weakest defensible basis for a decision.
Even the best single method — a structured interview at .42 — leaves most of the variation in performance unexplained. The way to predict better is to combine methods that measure different things, so each adds information the other misses. That added prediction is called incremental validity.
The classic evidence. In Schmidt & Hunter (1998), pairing a cognitive-ability test with a second method produced the strongest composites: GMA plus a work-sample test and GMA plus a structured interview each reached a composite validity of about .63, and GMA plus an integrity test about .65. The exact magnitudes are pre-2022 and now too high (cognitive ability’s own validity was revised down to .31), but the principle holds and is reinforced by the newer work: Sackett et al. (2023) show that well-chosen composites still substantially out-predict any single method, and that — because cognitive ability no longer dominates — interviews, work samples, biodata, and knowledge tests can be combined to reach strong validity without leaning on a cognitive test at all.
Why combining works. A structured interview captures interpersonal and judgement signals; a work sample or knowledge test captures task skill; a cognitive test captures learning speed. They correlate only modestly with each other, so each adds something. Two methods that measure the same thing add little; two that measure different things add a lot.
The cost of the interview-only hire. Most small employers decide on a single unstructured interview. That is the weakest defensible basis for a hire: unstructured-interview validity is about .19 (Sackett et al., 2022), it is the most bias-prone method, and it gives you no second, independent read on the candidate. Upgrading to a structured interview roughly doubles its validity; adding one more job-relevant method (a work sample for experienced hires, a knowledge test, or a structured reference verification) is the single biggest improvement most SMBs can make.
A sensible SMB stack. Structured interview + one job-relevant test (work sample or knowledge), with references used only to verify and red-flag at the end, as set out in the reference-checks note. This is cheap, high-validity, and — because every component is job-related and consistently applied — more defensible than a gut-feel conversation.
For an Ontario SMB, combining methods is also a risk-management move: a documented, multi-method, job-related process is both more accurate and easier to defend if a hiring decision is ever challenged under the rules in the Compliance cluster. The downside you are insuring against is quantified in the numbers cluster.
Source: Schmidt & Hunter, "The validity and utility of selection methods," Psychological Bulletin, 1998 ·
Last reviewed .
Confidence: Industry consensus
Related notes
- What actually predicts job performance: the selection-method validity hierarchy (post-2022) — Structured interviews, job-knowledge tests, empirically-keyed biodata, and work samples now predict job performance at or above general mental ability, whose operational validity was revised down from ~.51 to ~.31 in 2022 — so any ranking that still puts cognitive ability on top is outdated.
- Do structured interviews predict performance better than unstructured ones — and what makes an interview "structured"? — Structured interviews substantially out-predict unstructured ones — McDaniel et al. (1994) put corrected validity at about .44 vs .33, and Sackett et al. (2022) at .42 vs .19 — and "structure" means job-analysis-based questions asked identically of every candidate, scored on anchored rating scales by trained raters.
- Work-sample, ability, and job-knowledge tests: how well they predict and when to use them — Job-knowledge tests (.40) and work samples (.33) are strong, job-specific predictors and now sit at or above cognitive ability (.31) — but work-sample validity was revised down sharply in 2022, work samples and knowledge tests only work for candidates who already have the skills, and cognitive tests carry the largest adverse-impact risk.
- Which hiring / selection methods actually predict job performance? — On the best current peer-reviewed evidence (Sackett, Zhang, Berry & Lievens, 2022), structured interviews are the single strongest predictor of job performance (operational validity ≈ .42), ahead of job-knowledge tests (.40), empirically-keyed biodata (.38), work samples (.33), and general mental ability/cognitive tests (.31) — a major reordering from Schmidt & Hunter's widely-cited 1998 ranking, which over-stated cognitive ability at .51 due to range-restriction overcorrection.