How often does a team finish first one season, then sink to last the next? And how often does the opposite happen, with a last-place team improving to top the table?

In the N.F.L., it happens all the time.

Whether it is because of the short 16-game schedule or the likelihood of injuries to key players, the N.F.L. is a volatile league, year to year. In each of the last four seasons, and in eight of the last nine, at least one team that topped its division (or tied for first) the previous season slipped to last place the next. And at least one team that finished last wound up first.

Last season, the Jacksonville Jaguars ended at 5-11, completing a single-season reversal after finishing 10-6 — and four points short of a Super Bowl berth — in 2017. By contrast, the Houston Texans (11-5) and Chicago Bears (12-4) each went from last place to topping their respective divisions, both improving by seven wins.

That was a quiet year compared to the tumult of 2010, when four first-place teams — the Cincinnati Bengals, Dallas Cowboys, Minnesota Vikings and Arizona Cardinals — collapsed to last and two — the Texans and Denver Broncos — climbed to first.