# Formula for standardized root mean square residual (SRMR) in longitudinal latent variable models (SEM, CFA)

I was wondering what the adaption to the formula should be in case of multiple groups or longitudinal data (i.e. several time points).  As provided in the related / linked question, source: Hu, L.; Bentler, Peter (1999). "Cutoff criteria for fit indexes in covariance structure analysis: Conventional criteria versus new alternatives". Structural Equation Modeling. 6 (1): 1–55. https://dx.doi.org/10.1080%2F10705519909540118

Demonstration for calculating SRMR for two groups: $$SRMR = \sqrt{\frac{n_1·SRMR_1^2 + n_2·SRMR_2^2}{n_1+n_2}}$$ where $n_i$ and $SRMR_i$ are the sample size and $SRMR$ of group $i$, respectively. (Worked example in comments below.)
• If you have two groups, $n_1 = 200$, $SRMR_1 = 0.04$, $n_2 = 500$, $SRMR_2 = 0.02$, then $SRMR = \sqrt{\frac{n_1SRMR_1^2 + n_2SRMR_2^2}{n_1+n_2}}=\sqrt{\frac{200·(0.04)^2 + 500·(0.02)^2}{200+400}} = \sqrt{\frac{0.52}{700}} = .0273$. – Gregg H Apr 22 '18 at 14:03