On the arithmetic of mystery mixes
The question arrives with every shelter puppy: how big is this dog actually going to be? The honest answer is a range, and the honest range comes from a chart. When a pediatrician wonders about a child, she does not consult her intuition; she opens a growth chart and reads the family of curves the child is traveling among. Dogs, it turns out, grow along curves every bit as orderly.
The curves on a PredictPet chart are percentiles. Read the small numbers at the ends of the lines: a puppy riding the 50th line is dead average for the adult weight we expect, while the 2nd and the 98th fence in nearly everything that plausibly happens. Confidence, drawn honestly, is not one thick stripe but a family of thin lines, and the width of the family is the width of our knowing.
A known breed pulls the family close together, because many thousands of Labradors have been weighed before yours. A mystery mix starts wide. If you met the mother, her breed and weight tug the middle of the family toward her; if you never met either parent, the chart admits as much and opens its arms.
The two dates under the chart have exact meanings, and it is worth saying them plainly. Half grown is the day the middle curve of the family crosses half of the adult weight we predict — a date sharp enough to write with a day attached. Full grown is the day that curve crosses ninety-five percent of the predicted adult weight, where growth visibly flattens even though many dogs keep filling out a little after. Because the plateau arrives gradually, that second date is honest only to the month, and that is how we print it — never with a day.
Every weigh-in you write into the table re-fits the whole family. Two entries are better than one, and five are better than two — by the third, most puppies have chosen a lane and keep to it. This is also the chart’s built-in honesty: if your pup leaves the band, the band moves, and the milestone dates move with it. Expect those dates to firm up the way a pencil line is gone over in ink.
The mathematics underneath is not ours. The growth standards were fit to the published growth charts of the Waltham Centre’s longitudinal study of healthy dogs, published by Salt and colleagues in 2017, which did for puppies what the World Health Organization’s standards did for children. Our charts borrow the WHO’s printed grammar on purpose: the grid, the labeled family of curves, the little table waiting in the corner. A prediction you can check is worth keeping, and a prediction you can keep is the first chapter of a biography.