Table 1.

Sources of Unreliability and Their Effects.

	Reliability Indicator Affected?
	Absolute		Differential		Validity Affected?
Source	Alpha	Retest	Alpha	Retest	Absolute	Differential
Item Irrelevance	Yes	No	Yes	No	Yes	Yes
Item Heterogeneity	Yes	No	Yes	No	?	?
State Variation	No	Yes	No	Yes	Yes	Yes
Respondents' Error	Yes	Yes	No	No	Yes	No
Item Ambiguity	Yes	Yes	Yes	Yes	Yes	Yes
Sample Variance	Yes	Yes	Yes	Yes	Yes	Yes

Note. This table summarizes conceptual arguments offered in the text, not empirical findings. Absolute = magnitude for a given test compared to a fixed standard (e.g., .70); Differential = magnitude compared to that of other tests administered to the same sample. Item irrelevance = inappropriateness of item content. Item heterogeneity = statistical independence of trait indicators. State variation = effects of time-of-administration. Respondents' error = item misunderstanding or carelessness. Item ambiguity = lack of clarity or intelligibility of items; also includes effects due to the interaction of respondent and item characteristics. Sample variability = range of variation of traits in the sample. ? = Effect unknown.