ss.power.pcm: Sample size planning for power for polynomial change models

In MBESS: The MBESS R Package

Sample size planning for power for polynomial change models

Description

Returns power given the sample size, or sample size given the desired power, for polynomial change models (currently only linear, that is, straight-line, change models)

Usage

ss.power.pcm(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 coefficient for the group by time (linear) 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 measurements 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	total sample size (one-half in each of the two groups)
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; [e-mail])

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.pcm(beta=-.4, tau=.003, level.1.variance=.0262, frequency=2, duration=2, 
desired.power=.80, alpha.level=.05, standardized=TRUE, directional=FALSE)
ss.power.pcm(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.pcm(beta=-.0219, tau=.003, level.1.variance=.0262, frequency=2, duration=2, 
# desired.power=.80, alpha.level=.05, standardized=FALSE, directional=FALSE)
ss.power.pcm(beta=-.0219, tau=.003, level.1.variance=.0262, frequency=2, duration=2, 
N=238, alpha.level=.05, standardized=FALSE, directional=FALSE)

MBESS documentation built on Oct. 26, 2023, 9:07 a.m.