"U.S. teachers take off an average of 9.4 days (roughly 1 day per month) each during a typical 180-day school year," June Kronholz writes in Education Next. "By that estimate, the average child has substitute teachers for more than six months of his school career."
Duke University researchers Charles Clotfelter, Helen Ladd, and Jacob Vigdor "found that being taught by a sub for 10 days a year has a larger effect
on a child’s math score than if he’d changed schools, and about half the
size of the effect of poverty," Kronholz adds. "Columbia researchers Mariesa Herrmann
and Jonah Rockoff concluded that the effect on learning of using a
substitute for even a day is greater than the effect of replacing an
average teacher with a terrible one, that is, a teacher in the 10th
percentile for math instruction and the 20th percentile in English