Which criterion would NOT contribute to defining reliability in a measurement instrument?

The criterion that would not contribute to defining reliability in a measurement instrument is variation. Reliability in measurement instruments primarily refers to the consistency, stability, and equivalence of the results obtained when the instrument is used.

Consistency involves the instrument producing similar results across multiple trials or instances; this reflects how reliable the measurement can be when applied in the same context. Stability over time means that the instrument yields the same results when administered at different times, given that the variable being measured has not changed. Additionally, equivalence between different forms of the instrument (known as parallel forms reliability) ensures that various versions of the same test, when used interchangeably, yield similar results.

Variation, on the other hand, refers to the differences or discrepancies in the data collected, whether caused by the instrument itself or the variability in the subjects being measured. Therefore, while some variation may exist in data due to individual differences or other factors, it does not contribute to understanding the reliability of the measurement instrument directly. Instead, it is the goal of reliable measurement to minimize this variation to create consistent and stable results across different conditions and populations.