Reduce item overlap between forms
The advice. Offered in answer to OEE’s direct question, “What should be the overlap?” The committee named a target band roughly half the ceiling in OEE’s working definition, with the qualification that it “has to be in association with the other variables; you might have to adjust number of forms based upon percentage of overlap.”
Why. Halving the ceiling is what makes “meaningfully different” mean anything — see Define the inference the MTTC actually defends for why the phrase currently has no criterion. The qualification matters as much as the target: overlap is not independently settable. It trades against how many forms a field can sustain, which trades against examinee volume and the 100-examinee equating threshold.
The specific target band, and the working-definition ceiling it halves, are test-security sensitive and are held in the restricted layer — see restricted/mttc-form-security-detail.md.
Companion. Prefer fewer forms with less overlap states the direction of that trade.
Status. Open.
Related. MTTC Meaningfully Different Forms · Define ‘meaningfully different’ per incidence group