Functional Size
Definition
The number of lineup members who are plausible perpetrators given the witness's prior description. A lineup with nominal size six but functional size one is effectively a showup, regardless of physical population.
- Measures
- Number of plausible-perpetrator lineup members
- Contrast
- Nominal size, the physical headcount
- Failure case
- Functional size one behaves like a showup
Common questions
Why can a lineup with six people still be unfair?+
If five fillers clearly do not match the witness's description, such as differing in age, build, or a distinctive feature, the witness can eliminate them at a glance and the suspect stands out by default, so the lineup functions the same as presenting one person alone.
How do investigators keep functional size close to nominal size?+
Fillers are selected to match the witness's prior verbal description of the perpetrator, not just to resemble the suspect, and independent reviewers or mock-witness studies can check beforehand whether the suspect stands out disproportionately.
Related terms
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- Double-Blind Administration
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- Must-Be-There Bias
- The tendency of witnesses to make a lineup identification even when uncertain, if they believe the perpetrator is present. Substantially reduced by...
- Relative Judgement
- The cognitive strategy of comparing lineup members to each other and selecting the best match to memory. Encouraged by the simultaneous format...
- Sequential Lineup
- A lineup in which members are presented one at a time, with the witness making a yes/no decision on each before seeing...
- Simultaneous Lineup
- A lineup in which the witness views all persons or photographs at the same time and selects one. Encourages relative judgement (who...
- VIPER System
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