proximal.med {QuantPsyc} | R Documentation |
Simple Mediation Models
Description
Computes the Indirect Effect for a simple 3 variable mediation model: X -> M -> Y assuming direct effect X -> Y
Usage
proximal.med(data)
Arguments
data |
data.frame containing the variables labeled 'x', 'm', and 'y' respectively. |
Details
This function computes all paths in the simple 3 variable system involving the following regressions:
Y = t'X + bM, and
M = aX
where t' + ab = t
The indirect effect is computed as the product of a*b. Several formula are used for the computation of the standard error for the indirect effect (see MacKinnon et al for a comprehensive review).
As noted below, one can use this function to create the indirect effect and then utilize bootstrapping for a more accurate estimate of the standard error and model the distribution of the direct effect.
Value
Creates a table containing the following effects, their standard errors, and t-values :
a |
Effect of X on M |
b |
Effect of M on Y controlling for X |
t |
Total effect of X on Y |
t' |
Direct effect of X on Y accounting for M |
ab |
Indirect effect of X on Y though M |
Aroian |
Standard error of ab using Aroian method |
Goodman |
Standard error of ab using Goodman method |
Med.Ratio |
Mediation Ratio: indirect effect / total effect |
Warning
This function is primative in that it is based on a simplistic model AND forces the user to name the variables in the dataset x, m, and y.
Note
This function uses the following undocumented functions: aroian.se.indirect2
, goodman.se.indirect2
, se.indirect2
Author(s)
Thomas D. Fletcher t.d.fletcher05@gmail.com
References
MacKinnon, D. P., Lockwood, C. M., Hoffman, J. M., West, S. G., & Sheets, V. (2002). A comparison of methods to test mediation and other intervening variable effects. Psychological Methods, 7, 83-104.
See Also
Examples
data(tra)
tmp.tra <- tra
names(tmp.tra) <- c('x','z','m','y')
data.frame(proximal.med(tmp.tra)) ## data.frame() simple makes the table 'pretty'