Background: Reliability studies are commonly used in questionnaire development studies
and questionnaire validation studies. This study reviews the sample size guideline for Cronbach’s
alpha test.
Methods: Manual sample size calculation using Microsoft Excel software and sample size
tables were tabulated based on a single coefficient alpha and the comparison of two coefficients
alpha.
Results: For a single coefficient alpha test, the approach by assuming the Cronbach’s alpha
coefficient equals to zero in the null hypothesis will yield a smaller sample size of less than 30 to
achieve a minimum desired effect size of 0.7. However, setting the coefficient of Cronbach’s alpha
larger than zero in the null hypothesis could be necessary and this will yield larger sample size. For
comparison of two coefficients of Cronbach’s alpha, a larger sample size is needed when testing for
smaller effect sizes.
Conclusions: In the assessment of the internal consistency of an instrument, the present
study proposed the Cronbach’s alpha’s coefficient to be set at 0.5 in the null hypothesis and
hence larger sample size is needed. For comparison of two coefficients’ of Cronbach’s alpha,
justification is needed whether testing for extremely low and extremely large effect sizes are
scientifically necessary.