VS (Variable System) is a free open-source statistical package for the analysis of conditional path models based on structural equation modeling (SEM). It can be used to examine models with simple mediation, simple moderation, moderated mediation (moME), mediated moderation (meMO), and any combinations of them.

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VS is implemented as a R package and runs in the R system for statistical computing (R Core Team, 2022). In other words, you need to install R in order to use the VS package. 

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VS is easy to use. Users only need to specify the conceptual model using simple control languages in R and the package will automatically (1) transform the conceptual model into a testable SEM-based model, (2) estimate the basic model parameters, (3) compute and test the conditional effects of interest, and (4) evaluate the model goodness-of-fit. 

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Kwan, J. L.-Y., & Chan, W. (2018). Variable System: An alternative approach for the analysis of mediated moderation. Psychological Methods, 23(2), 262-277 .  https://doi.org/10.1037/met0000160