What is the meaning of a negative Standardized mean difference (SMD)? The core of my question is: What is the meaning of a negative SMD?
To assess the balance after Propensity Score Matching (PSM) with the R package MatchIt the documentation explains the Std. Mean Diff (Standardized mean difference).
The values in that example are positive and negativ. My problem is that there are also negative.
Because Linden (2013) explained that the SMD could have cut-offs from 0.10 to 0.25. This means a value higher then 0.10 (or 0.25; where ever you set your cut off) means imbalance.
But Linden never wrote about negativ values.
This is the formula Linden used in the publication:


Standardized mean differences are not always presented in absolute value. In MatchIt, they are not. A negative SMD just means the control group has a larger mean the the treated group, and a positive SMD just means the treated group has a larger mean than the control group. The cutoffs refer to the absolute value of the SMD, but an SMD of -.15 has the same degree of imbalance as an SMD of .15.
