Interactions in the blip down function don't work

[mattblackwell]

MWE:

library(DirectEffects)

# ACDE of the plow on female participation, with income as mediator
form_main <- women_politics ~ plow + rugged | oil_pc | centered_ln_inc + centered_ln_inc:plow

direct2 <- sequential_g(formula = form_main, data = ploughs)
#> Error in XZM[, mnames, drop = FALSE]: subscript out of bounds

^{Created on 2019-01-03 by the reprex package (v0.2.1)}

[kuriwaki]

^ edited original post to a reprex

[kuriwaki]

just documenting a bit more --

This breaks at

DirectEffects/R/DirectEffects.R

Line 137 in f1ee624

M <- XZM[, mnames, drop = FALSE]

where colnames(XZM) is

[1] "(Intercept)"   "plow"    "rugged"       "oil_pc"              
[5] "centered_ln_inc"      "plow:centered_ln_inc"

and mnames is

[1] "centered_ln_inc"    "centered_ln_inc:plow"

so at least in this case, it's just the ordering of the interaction term that is the problem.

going back further, XZM is made from the entire formula:

DirectEffects/R/DirectEffects.R

Lines 127 to 134 in f1ee624

	## add to mf call
	mf$formula <- formula
	# finally evaluate model.frame, create data matrix
	mf <- eval(mf, parent.frame())
	mt <- attr(mf, "terms") # terms object
	XZM <- model.matrix(mt, mf, contrasts)

probably model.matrix swaps the ordering as plow:centered_ln_inc because the model frame is built without interactions and there plow shows up first:

> colnames(mf)
[1] "women_politics"  "plow"  "rugged"   "oil_pc"  "centered_ln_inc"

In contrast mnames is built from the only the third part of the formula RHS, which captures the original centered_ln_inc:plow notation

DirectEffects/R/DirectEffects.R

Lines 124 to 125 in f1ee624

	mt_m <- terms(formula, data = data, rhs = 3)
	mnames <- attr(mt_m, "term.labels")

[kuriwaki]

Therefore, predictably, in these alternate specifications of the formula,

library(DirectEffects)
#> Warning: package 'DirectEffects' was built under R version 3.5.2

form_alt1 <- women_politics ~ plow + rugged | oil_pc | centered_ln_inc + plow:centered_ln_inc
sequential_g(form_alt1, data = ploughs)
#> Error in XZM[, mnames, drop = FALSE]: subscript out of bounds

form_alt2 <- women_politics ~ plow + rugged | oil_pc | plow:centered_ln_inc  + centered_ln_inc
sequential_g(form_alt2, data = ploughs)
#> 
#> Call:
#> sequential_g(formula = form_alt2, data = ploughs)
#> 
#> 
#> Coefficients:
#> (Intercept)        plow      rugged 
#>  13.3242026  -1.3964834  -0.5526104

^{Created on 2019-01-09 by the reprex package (v0.2.1)}

The former of the two still does not work because now it is mnames that reorders plow:centered_ln_inc to centered_ln_inc:plow internally, which does not match with the colnames(XZM). However the latter does work, because plow comes up first before centered_ln_inc, just like it does in the entire formula.

A quick Google Search of variable orderings within an interaction term doesn't reveal any obvious fixes.