Baseline balance is important for both experiments and quasi-experiments, just not in the way researchers sometimes believe. Here are excerpts from three of my favourite discussions of the topic.
Don’t test for baseline imbalance in RCTs. Senn (1994, p. 1716):
“… the following are two incontrovertible facts about a randomized clinical trial:
1. over all randomizations the groups are balanced;
2. for a particular randomization they are unbalanced.
Now, no ‘[statistically] significant imbalance’ can cause 1 to be untrue and no lack of a significant balance can make 2 untrue. Therefore the only reason to employ such a test must be to examine the process of randomization itself. Thus a significant result should lead to the decision that the treatment groups have not been randomized…”
Do examine baseline imbalance in quasi-experiments; however, not by using statistical tests. Sample descriptives, such as a difference in means, suffice. Imai et al. (2008, p. 497):
“… from a theoretical perspective, balance is a characteristic of the sample, not some hypothetical population, and so, strictly speaking, hypothesis tests are irrelevant…”
Using p-values from t-tests and similar can lead to erroneous decisions of balance. As you prune a dataset to improve balance, power to detect effects decreases. Imai et al. (2008, p. 497 again):
“Since the values of […] hypothesis tests are affected by factors other than balance, they cannot even be counted on to be monotone functions of balance. The t-test can indicate that balance is becoming better whereas the actual balance is growing worse, staying the same or improving. Although we choose the most commonly used t-test for illustration, the same problem applies to many other test statistics…”
If your matching has led to baseline balance, then you’re good, even if the matching model is misspecified. (Though not if you’re missing key covariates, of course.) Rosenbaum (2023, p. 29):
“So far as matching and stratification are concerned, the propensity score and other methods are a means to an end, not an end in themselves. If matching for a misspecified and misestimated propensity score balances x, then that is fine. If by bad luck, the true propensity score failed to balance x, then the match is inadequate and should be improved.”
Imai, K., King, G., & Stuart, E. A. (2008). Misunderstandings between experimentalists and observationalists about causal inference. Journal of the Royal Statistical Society: Series A (Statistics in Society), 171(2), 481–502.
Rosenbaum, P. R. (2023). Propensity score. In J. R. Zubizarreta, E. A. Stuart, D. S. Small, & P. R. Rosenbaum, Handbook of Matching and Weighting Adjustments for Causal Inference (pp. 21–38). Chapman and Hall/CRC.
Senn, S. (1994). Testing for baseline balance in clinical trials. Statistics in Medicine, 13, 1715–1726.