An impact-evaluation workflow

This vignette walks from a study’s raw data to a baseline-equivalence report, using the bundled (simulated) tutoring dataset.

library(baselinr)
data(tutoring)
head(tutoring)
#>   treat  pretest attendance  age female frpl ell posttest
#> 1     1 33.02273  0.9416614 10.2      1    0   1     49.8
#> 2     1 47.86310  0.9759478 10.0      1    0   1     53.0
#> 3     1 42.15184  1.0000000 10.3      0    0   0     49.2
#> 4     1 45.37557  0.9483293 10.7      1    0   0     57.4
#> 5     1 38.89146  0.9512494  9.4      0    1   0     49.3
#> 6     1 47.94922  0.9757792  9.4      0    0   1     43.8

tutoring is a simulated quasi-experimental evaluation: 200 students who received a tutoring program (treat = 1) and 200 who did not (treat = 0), with baseline covariates and a post-program posttest.

Step 1: assess baseline equivalence, before looking at the outcome

The credibility of any later effect estimate rests on whether the two groups were comparable at baseline. We pass the baseline covariates explicitly. We do not include posttest, which is an outcome, not a baseline covariate.

baseline_covs <- c("pretest", "attendance", "age", "female", "frpl", "ell")

equiv <- baseline_equivalence(tutoring, treatment = "treat",
                              covariates = baseline_covs)

knitr::kable(equiv, digits = 3)
covariate type n_treatment n_comparison mean_treatment mean_comparison sd_treatment sd_comparison effect_size wwc_category
pretest continuous 200 200 52.150 50.099 9.824 10.993 0.196 satisfied_with_adjustment
attendance continuous 200 200 0.927 0.918 0.053 0.049 0.186 satisfied_with_adjustment
age continuous 200 200 10.044 10.034 0.508 0.504 0.020 satisfied
female binary 200 200 0.530 0.525 0.500 0.501 0.012 satisfied
frpl binary 200 200 0.555 0.445 0.498 0.498 0.268 not_satisfied
ell binary 200 200 0.250 0.150 0.434 0.358 0.385 not_satisfied

baselinr automatically uses Hedges’ g for the continuous covariates (pretest, attendance, age) and the Cox index for the binary ones (female, frpl, ell).

Step 2: read the categories

Each covariate falls into one of three What Works Clearinghouse categories:

equiv[, c("covariate", "effect_size", "wwc_category")]
#>    covariate effect_size              wwc_category
#> 1    pretest  0.19639834 satisfied_with_adjustment
#> 2 attendance  0.18641935 satisfied_with_adjustment
#> 3        age  0.01971405                 satisfied
#> 4     female  0.01215809                 satisfied
#> 5       frpl  0.26775010             not_satisfied
#> 6        ell  0.38544774             not_satisfied
  • satisfied: the groups are equivalent on this covariate; nothing more to do.
  • satisfied_with_adjustment: equivalence holds only if you statistically adjust for this covariate in the impact model. This is a commitment, not a pass: those covariates must appear in the model.
  • not_satisfied: this covariate cannot establish equivalence even with adjustment. It’s a threat to the study’s credibility that you have to confront, not bury.

Step 3: visualise

love_plot(equiv)

The dashed lines mark the 0.05 and 0.25 thresholds; points are coloured by category. The plot makes the at-risk covariates obvious at a glance.

Step 4: a report-ready table

For a written report or a Quarto/HTML document, gt_baseline() returns a formatted gt table:

gt_baseline(equiv)

What this does and doesn’t tell you

baselinr reports the baseline equivalence picture. It does not fit the impact model for you. The next steps are yours: include the satisfied_with_adjustment covariates in the model, and decide how to handle (or report the limitation of) any not_satisfied covariate before you interpret the program’s effect on posttest.