Demographers once had a clever method for estimating the Jewish population.
They would ask school principals how many students had been absent on Yom Kippur, the Day of Atonement, when synagogue attendance is at its highest. With a few quick calculations, that figure could help provide a rough and imperfect estimate of the total Jewish population in a given neighborhood.
The Hebrew Institute used this method throughout the late 1940s, as the local Jewish population was consolidating to Squirrel Hill, Oakland, and the East End, after decades of expanding throughout the city and the wider region.