A subgroup effect is not the full-sample effect restricted to a few
rows: the target changes, so the clever covariate changes with it.
Conditioning on S = 1 divides the efficient influence function
by P(S = 1) and zeroes it outside the subgroup, giving
H = I(S = 1)/P(S = 1) * [D/g(X) - (1 - D)/(1 - g(X))].
Details
The nuisance models are fitted on the whole sample – the point of not
subsetting first – while the target parameter is the subgroup one.
With S all ones this reduces exactly to the full-sample TMLE.
References
Chernozhukov, V., Demirer, M., Duflo, E. & Fernandez-Val, I. (2025). Econometrica 93(4):1121-1164; van der Laan, M. J. & Rubin, D. (2006). IJB 2(1):11.
Examples
Tmlsbg(y = c(1, 2, 3, 4, 5, 6, 7, 8), D = c(1, 2, 3, 4, 5, 6, 7, 8),
X = c(1, 2, 3, 4, 5, 6, 7, 8), subgroup = c(1, 2, 3, 4, 5, 6, 7, 8))
#> $estimate
#> [1] -1.346696e+16
#>
#> $se
#> [1] 6.380237e+17
#>
#> $eps
#> [1] -1.172026e+14
#>
#> $n_sub
#> [1] 36
#>
#> $n
#> [1] 8
#>
#> $method
#> [1] "TMLE for the average treatment effect within a subgroup"
#>
#> attr(,"class")
#> [1] "morie_rich_result" "list"
