cronbach.alpha.CI {cocron} | R Documentation |
Calculates a confidence interval for Cronbach's alpha (Cronbach, 1951).
cronbach.alpha.CI(alpha, n, items, conf.level = 0.95)
alpha |
A numeric specifying the alpha coefficient. |
n |
A numeric defining the number of individuals who provided the data for the test for which the alpha coefficient was determined. |
items |
A numeric specifying the number of items the alpha coefficient is based on. |
conf.level |
A number defining the level of confidence for the confidence interval (default is .95). |
The lower bound of a confidence interval for an α that is based on the data of n individuals who responded to k items is defined as
L = 1 - ((1 - α) F(1 - c/2))
where c is the level of confidence and F(1 - c/2) the 100(1 - c/2) percentile of the F-distribution with df_1 = n - 1 and df_2 = (n - 1)(k - 1) (Feldt, Woodruff, & Salih, 1987, p. 95, formula 6). The upper bound of the confidence interval is computed as
U = 1 - ((1 - α) F(c/2))
(Feldt et al., 1987, p. 95, formula 7).
Returns a confidence interval for Cronbach's alpha as a numeric vector.
Cronbach, L. J. (1951). Coefficient alpha and the internal structure of tests. Psychometrika, 16, 297-334.
Feldt, L. S., Woodruff, D. J., & Salih, F. A. (1987). Statistical inference for coefficient alpha. Applied Psychological Measurement, 11, 93-103.
cronbach.alpha.CI(alpha=.83, n=100, items=20, conf.level=.95)