Unreliable NC estimation of the heterogeneity of an interaction from observations under two conditions. Left: An interaction with a coefficient vector, (a, b), which might shift under two conditions; Center: The observations, (O1, O2), led to estimation of a large, but insignificant, difference between the coefficient vectors. (P1, P2) are model predictions; Right: The observations, (O1, O2), let to estimation of a small, but significant, difference between the coefficient vectors.(P1, P2) are model predictions.