ss.power.lrd {MBESS}R Documentation

Sample size planning for power for a longitudinal randomized straight-line change model

Description

Returns power given the sample size, or sample size given the desired power, for a longitudinal randomized design.

Usage

ss.power.lrd(beta, tau, level.1.variance, frequency, duration, desired.power = NULL, N = NULL, alpha.level = 0.05, standardized = TRUE, directional = FALSE)

Arguments

beta the level two regression coefficeint for the group by time interaction; where "X" is coded -.5 and .5 for the two groups.
tau the true variance of the individuals' slopes
level.1.variance level one variance
frequency frequency of measruemetns per unit of time duration of the study in the particular units (e.g., age, hours, grade level, years, etc.)
duration time in some number of units (e.g., years)
desired.power desired power
N sample size
alpha.level Type I error rate
standardized the standardized slope is the unstandardized slope divided by the square root of tau, the variance of the unique effects for beta.
directional should a one (TRUE) or two (FALSE) tailed test be performed.

Author(s)

Ken Kelley (University of Notre Dame; KKelley@ND.Edu)

References

Raudenbush, S. W., & X-F., Liu. (2001). Effects of study duration, frequency of observation, and sample size on power in studies of group differences in polynomial change. Psychological Methods, 6, 387–401.

Examples

# Example from Raudenbush and Liu (2001)
# ss.power.lrd(beta=-.4, tau=.003, level.1.variance=.0262, frequency=2, duration=2, desired.power=.80, alpha.level=.05, standardized=TRUE, directional=FALSE)
# ss.power.lrd(beta=-.4, tau=.003, level.1.variance=.0262, frequency=2, duration=2, N=238, alpha.level=.05, standardized=TRUE, directional=FALSE)

# The standardized effect size is obtained as beta/sqrt(tau): -.4/sqrt(.003) = -.0219.
# ss.power.lrd(beta=-.0219, tau=.003, level.1.variance=.0262, frequency=2, duration=2, desired.power=.80, alpha.level=.05, standardized=FALSE, directional=FALSE)
# ss.power.lrd(beta=-.0219, tau=.003, level.1.variance=.0262, frequency=2, duration=2, N=238, alpha.level=.05, standardized=FALSE, directional=FALSE)


[Package MBESS version 2.0.0 Index]