## Cronbach alpha coefficient

Cronbach’s alpha reliability coefficient normally ranges between 0 and 1. However, there is actually no lower limit to the coefficient. The closer Cronbach’s alpha coefficient is to 1.0 the greater the internal consistency of the items in the scale.

Based upon the formula _ = rk /[1 + (k -1)r] where k is the number of items considered and r is the mean of the inter-item correlations the size of alpha is determined by both the number of items in the scale and the mean inter-item correlations. George and Mallery (2003) provide the following rules of thumb:

“_ > .9 – Excellent,

_ > .8 – Good,

_ > .7 – Acceptable,

_ > .6 – Questionable,

_ > .5 – Poor, and

_ < .5 – Unacceptable” (p. 231).

While increasing the value of alpha is partially dependent upon the number of items in the scale, it should be noted that this has diminishing returns. It should also be noted that an alpha of .8 is probably a reasonable goal. It should also be noted that while a high value for Cronbach’s alpha indicates good internal consistency of the items in the scale, it does not mean that the scale is unidimensional.

When using Likert-type scales it is imperative to calculate and report Cronbach’s alpha coefficient for internal consistency reliability for any scales or subscales one may be using. The analysis of the data then must use these summated scales or subscales and not individual items. If one does otherwise, the reliability of the items is at best probably low and at worst unknown.

Cronbach’s alpha does not provide reliability estimates for single items.

## Alpha cronbach formula

Cronbach’s α is defined as where K is the number of components (K-items or testlets),  the variance of the observed total test scores, and  the variance of component i for the current sample of persons. See Develles (1991). Alternatively, the Cronbach’s α can also be defined as where K is as above,  the average variance, and  the average of all covariances between the components across … Continue reading

## Cronbach’s Alpha

Cronbach’s α (alpha) is a coefficient of reliability. It’s frequently used as a measure of the internal consistency or reliability of a psychometric test score for a sample of examinees. It was initially named alpha by Lee Cronbach in 1951, as he had intended to continue with further coefficients. The measure can be viewed an … Continue reading