What does a negative standardized mean difference suggest about treatment and control?

A negative standardized mean difference indicates that the mean score of the treatment group is lower than that of the control group. This suggests that the control condition is better than the treatment for the outcome being measured. In essence, the treatment's effect is detrimental or less beneficial compared to the control group's effect.

When interpreting the standardized mean difference, it's important to recognize that the context and the specific outcomes being measured are critical. Here, the negative value signifies that the intervention or treatment did not yield a superior result; rather, it performed worse than no treatment or the comparison group. In research terms, this can imply that, at least within the parameters of the study, the control group demonstrated more effectiveness or more favorable outcomes relative to the treatment group.

While it is possible that further data could provide additional insights or nuances, the result of the negative standardized mean difference itself is clear in suggesting that the control group outperformed the treatment group.