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The Nets’ superstar Kevin Durant will soon grow by two to three inches, while Kemba Walker of the Boston Celtics, J.J. Barea of the Dallas Mavericks and others are likely to shrink by a similar amount.

The listed heights of N.B.A. players have long been widely considered more art than science, but the league notified teams this week that they are required to certify and submit each player’s height and age within the first week of training camp, according to four people familiar with the new policy.

Sparked by the league’s determination to emphasize “the integrity of information” it disseminates publicly, front offices were informed that player heights must be certified by a team physician and their ages confirmed via the submission to the league office of a driver’s license or passport, according to the people, who were not authorized to discuss the procedural changes publicly.

Among the motivations for the crackdown is the case of the Sacramento Kings’ Buddy Hield, whose age was discovered in December 2018 to have been recorded incorrectly.