Background: We aimed to assess the impact of inconsistent responses on the internal reliability of a multi-item scale by developing a procedure to adjust Cronbach's alpha.
Methods: A procedure for adjusting Cronbach's alpha when there are inconsistent responses was developed and used to assess the impact of inconsistent responses on internal reliability by evaluating the standard Chinese 12-item Short Form Health Survey in adolescents.
Results: Contrary to common belief, random responses may inflate Cronbach's alpha when their mean differ from that of the true responses. Fixed responses inflate Cronbach's alpha except in scales with both positive and negative polarity items. In general, the bias in Cronbach's alpha due to inconsistent responses may change from negative to positive with an increasing number of items in a scale, but the effect of additional items beyond around 10 becomes small. The number of response categories does not have much influence on the impact of inconsistent responses.
Conclusions: Cronbach's alpha can be biased when there are inconsistent responses, and an adjustment is recommended for better assessment of the internal reliability of a multi-item scale.