R help - Oct 2012 - two indirect effects of path analysis

Hello,

This is Elaine.

I am trying a path analysis using lavaan Package.

There are three explanatory variables: X, Z, and M.
The response variable is Y.
A, b, and c have direct effects on Y.

On the other hand, X and Z also have direct effects on M.
In other words, X and Z have indirect effects on Y.

I found the code example of lavaan package describes only one indirect
effect as below.
Please kindly advise how to modify it as two indirect effects.
Thank you.

Elaine
>  set.seed(1234)
>  X  <-  rnorm(100)
>  M  <-  0.5*X  +  rnorm(100)
>  Y  <-  0.7*M  +  rnorm(100)
>  Data  <-  data.frame(X  = X,  Y  =  Y,  M  =  M)
>  model  <-  '  #  direct  effect
+ Y  ~  c*X
+ #  mediator
+ M  ~  a*X
+ Y  ~  b*M
+ #  indirect  effect  (a*b)
+ ab  :=  a*b
+ #  total  effect
+ total  :=  c  +  (a*b)
+ '
>  fit  <-  sem(model,  data=Data)
>  summary(fit)
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On 10/07/2012 02:17 AM, Elaine Kuo wrote:
> Hello,
>
> This is Elaine.
>
> I am trying a path analysis using lavaan Package.
>
> There are three explanatory variables: X, Z, and M.
> The response variable is Y.
> A, b, and c have direct effects on Y.
>
> On the other hand, X and Z also have direct effects on M.
> In other words, X and Z have indirect effects on Y.
>
> I found the code example of lavaan package describes only one indirect
> effect as below.
> Please kindly advise how to modify it as two indirect effects.
You need to write down all the regressions that are involved in your 
model. For each 'dependent' variable, there is a regression formula:

model <- '
Y ~ X + Z + M
M ~ X + Z
'

Optionally, you can label the parameters, and define some indirect effects:

model <- '
Y ~ c1*X + c2*Z + b*M
M ~ a1*X + a2*Z

ab1 := a1*b
ab2 := a2*b

totalx := ab1 + c1
totalz := ab2 + c2
'

Hope this helps,

Yves.