With the exception of a student I knew in grad school (who unimaginatively fabricated a lab report so there was zero individual variability within each of two comparison groups), anyone remotely versed in statistics knows that there’s always variability around an average; and typically there are plenty of uncontrollable factors (i.e., individual differences) contributing to that variability.

So that’s why, as a parent, I bristle when I hear about mamas whose pediatricians, friends, or family have inspired panic and worry over their kid not walking, talking, or eating exactly on schedule with milestone charts. The same goes for arbitrary cutoff dates, such as kindergarten enrollment. Obviously there is utility and convenience to standards, but variability is not equivalent to failure; and clearly many parents think in terms of the latter.