WASHINGTON—Two leading experts on the “gig economy” now say their estimates of its impact were too high, skewed by spotty data and the recession of a decade ago.

Alan Krueger of Princeton University and Lawrence Katz of Harvard sifted through new evidence to explain how, in a 2015 survey, they overestimated how people cobbling together a living from odd jobs, especially via apps like Uber, would upend traditional work arrangements.

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