Computes mean(estimated) / mean(observed) as a measure of systematic
over- or under-prediction. A value of 1.0 indicates no bias. Pairs with
calc_model_R2(method = "corr") to give both pattern agreement and
magnitude calibration.
Value
Scalar ratio. Values > 1 indicate over-prediction, < 1 under-prediction.
Returns NA_real_ if observed mean is zero or no valid pairs exist.
