The reliability of a two-item scale: Pearson, Cronbach, or Spearman-Brown?
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To obtain reliable measures researchers prefer multiple-item questionnaires rather than single-item tests. Multiple-item questionnaires may be costly however and time-consuming for participants to complete. They therefore frequently administer two-item measures, the reliability of which is commonly assessed by computing a reliability coefficient. There is some disagreement, however, what the most appropriate indicator of scale reliability is when a measure is composed of two items. The most frequently reported reliability statistic for multiple-item scales is Cronbach’s coefficient alpha and many researchers report this coefficient for their two-item measure (Cuijpers et al. 2009; Löwe et al. 2005; Michal et al. 2010; Young et al. 2009). Others however claim that coefficient alpha is inappropriate and meaningless for two-item scales (Sainfort and Booske 2000; Verhoef 2003; Cramer et al. 2006; O’Brien et al. 2008). Instead, they recommend using the Pearson correlation...
KeywordsCoefficient Alpha True Score Local Dependence Classical Test Theory True Reliability
The authors are grateful to William Revelle and an anonymous reviewer for helpful comments on a previous version of this manuscript and suggestions for improvements.
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