ssMediation.VSMc {powerMediation}R Documentation

Sample size for testing mediation effect based on Vittinghoff and McCulloch's (2009) method

Description

Calculate sample size for testing mediation effect based on Vittinghoff and McCulloch's (2009) method.

Usage

ssMediation.VSMc(power, b2, sigma.m, sigma.e, corr.xm, 
  n.lower=1, n.upper=1e+30, alpha = 0.05, verbose=TRUE)

Arguments

power power for testing b_2=0 for the linear regression y_i=b0+b1 x_i + b2 m_i + \epsilon_i, \epsilon_i\sim N(0, \sigma_e^2).
b2 regression coefficient for the mediator m in the linear regression y_i=b0+b1 x_i + b2 m_i + \epsilon_i, \epsilon_i\sim N(0, \sigma_e^2).
sigma.m standard deviation of the mediator.
sigma.e standard deviation of the random error term in the linear regression y_i=b0+b1 x_i + b2 m_i + \epsilon_i, \epsilon_i\sim N(0, \sigma_e^2).
corr.xm correlation between the predictor x and the mediator m.
n.lower lower bound for the sample size.
n.upper upper bound for the sample size.
alpha type I error rate.
verbose logical. TRUE means printing sample size; FALSE means not printing sample size.

Details

The test is for testing the null hypothesis b_2=0 versus the alternative hypothesis b_2\neq 0 for the linear regressions:

y_i=b_0+b_1 x_i + b_2 m_i + \epsilon_i, \epsilon_i\sim N(0, \sigma^2_{e})

Vittinghoff et al. (2009) showed that for the above linear regression, testing the mediation effect is equivalent to testing the null hypothesis H_0: b_2=0 versus the alternative hypothesis H_a: b_2\neq 0.

Value

n sample size.
res.uniroot results of optimization to find the optimal sample size.

Note

The test is a two-sided test. Code for one-sided tests will be added later.

Author(s)

Weiliang Qiu stwxq@channing.harvard.edu

References

Vittinghoff, E. and Sen, S. and McCulloch, C.E.. Sample size calculations for evaluating mediation. Statistics In Medicine. 2009;28:541-557.

See Also

minEffect.VSMc, powerMediation.VSMc

Examples

  ssMediation.VSMc(power=0.8, b2=0.8, sigma.m=0.1, sigma.e=0.2, corr.xm=0.5, 
    alpha = 0.05, verbose = TRUE)

[Package powerMediation version 0.0.6 Index]