ss.aipe.reliability: Sample Size Planning for Accuracy in Parameter Estimation for... In MBESS: The MBESS R Package

Description

This function determines a necessary sample size so that the expected confidence interval width for the alpha coefficient or omega coefficient is sufficiently narrow (when assurance=NULL) or so that the obtained confidence interval is no larger than the value specified with some desired degree of certainty (i.e., a probability that the obtained width is less than the specified width; assurance=.85). This function calculates coefficient alpha based on McDonald's (1999) formula for coefficient alpha, also known as Guttman-Cronbach alpha. It also uses coefficient omega from McDonald (1999). When the 'Parallel' or 'True Score' model is used, coefficient alpha is calculated. When the 'Congeneric' model is used, coefficient omega is calculated.

Usage

 1 2 3 4 ss.aipe.reliability(model = NULL, type = NULL, width = NULL, S = NULL, conf.level = 0.95, assurance = NULL, data = NULL, i = NULL, cor.est = NULL, lambda = NULL, psi.square = NULL, initial.iter = 500, final.iter = 5000, start.ss = NULL, verbose=FALSE)

Arguments

 model the type of measurement model (e.g., "parallel items", "true-score equivalent", or "congeneric model") for a homogeneous single common factor test type the type of method to base the formation of the confidence interval on, either the "Factor Analytic" (McDonald, 1999) or "Normal Theory" (van Zyl, Neudecker, & Nel, 2000) width the desired full width of the confidence interval S a symmetric covariance matrix conf.level the desired confidence interval coverage, (i.e., 1- Type I error rate) assurance parameter to ensure that the obtained confidence interval width is narrower than the desired width with a specificied degree of certainty data the data set that the reliability coefficient is obtained from i number of items cor.est the estimated inter-item correlation lambda the vector of population factor loadings psi.square the vector of population error variances initial.iter the number of initial iterations or generations/replications of the simulation study within the function final.iter the number of final iterations or generations/replications of the simulation study start.ss the initial sample size to start the simulation at verbose shows extra information one the current sample size and current level of assurance; helpful if the function gets stuck in a long iterative process

Details

Use verbose=TRUE if the function is taking a very long time to provide an answer.

Value

 Required.Sample.Size the necessary sample size width the specified full width of the confidence interval specified.assurance the specified degree of certainty empirical.assurance the empirical assurance based on the necessary sample size returned final.iter the specified number of iterations in the simulation study

Warning

In some conditions, you may receive a warning, such as "In sem.default(ram = ram, S = S, N = N, param.names = pars, var.names = vars,; Could not compute QR decomposition of Hessian. Optimization probably did not converge." This indicates that the model likely did not converge. In certain conditions this may occur because the model is not being fit well due to small sample size, a low number of iterations, or a poorly behaved covariance matrix.

Note

Not all of the items can be entered into the function to represent the population values. For example, either 'data' can be used, or S, or i, cor.est, and psi.square, or i, lambda, and psi.square. With a large number of iterations (final.iter) this function may take considerable time.

Author(s)

Leann J. Terry (Indiana University; ljterry@Indiana.Edu); Ken Kelley (University of Notre Dame; KKelley@ND.Edu)

References

McDonald, R. P. (1999). Test theory: A unified approach. Mahwah, New Jersey: Lawrence Erlbaum Associates, Publishers.

van Zyl, J. M., Neudecker, H., & Nel, D. G. (2000). On the distribution of the maximum likelihood estimator of Cronbach's alpha. Psychometrika, 65 (3), 271–280.