`postResample`

is meant to be used with `apply`

across a matrix. For numeric data
the code checks to see if the standard deviation of either vector is zero. If so, the correlation
between those samples is assigned a value of zero. `NA`

values are ignored everywhere.Note that many models have more predictors (or parameters) than data points, so the typical mean squared
error denominator (n - p) does not apply. Root mean squared error is calculated using `sqrt(mean((pred - obs)^2`

.
Also, R-squared is calculated as the square of the correlation between the observed and predicted outcomes.

For `defaultSummary`

is the default function to compute performance metrics in `train`

. It is a wrapper around `postResample`

.

Other functions can be used via the `summaryFunction`

argument of `trainControl`

. Custom functions must have the same arguments as`defaultSummary`

.