Rilcon42 Rilcon42 - 1 year ago 236
R Question

`probemod::pickapoint` fails when reversing moderator and IV for a linear model with interaction

I am trying to model values for an interaction, but when I input the IV and moderator one way it fails. When I reverse them it works fine. Both the IV and moderator are numeric, so its not a data type issue. Anyone know why this is happening?

Error message:

Error in cov[interactionterms[j], interactionterms[j]] : subscript out of bounds


ppick<-pickapoint(lm.model, dv="mpg", iv="disp", mod="cyl", method='meansd')
ppick<-pickapoint(lm.model, dv="mpg", iv="cyl", mod="disp", method='meansd')

Answer Source

Note that your regression model is specified as cyl*disp, so the interaction term is cyl:disp. Order counts.

If you specify iv = "disp" and mod = "cyl", pickapoint will look for disp:cyl (trace the local variable interactionterms and you will see it) but there is no such term in lm.model.

Covariance matrix has dimnames:

#            (Intercept)          cyl         disp      cyl:disp
#(Intercept) 25.04638446 -3.770034504 -0.162461470  2.219922e-02
#cyl         -3.77003450  0.705917580  0.017132037 -2.807237e-03
#disp        -0.16246147  0.017132037  0.001600197 -1.929310e-04
#cyl:disp     0.02219922 -0.002807237 -0.000192931  2.448097e-05

therefore you get "subscript out of bounds" error when looking for disp:cyl.

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