# Group-level variances and correlations

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This is just a sum-up of what we should do when we try to estimate the group-level variances and correlations but the software failed to do so. As a R and lmer() user, I found this happens often.If this is the case, there are three potential problems:(1) sd is 0(2) some correlations are -1 or 1(3) covariance matrix is singularIf the problem is of (1), then we might as well forget about the
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