During the playoffs, broadcasters love to pull out fact-like objects based on incredibly small samples. Before Game 6 of the Eastern Conference finals, for example, announcers pointed out that, in the playoffs, Roy Hibbert had shot significantly better after making his first field-goal attempt. The impression this left was, "Oh boy, if Hibbert makes his first shot, it'll be a whole different ballgame, I tell you what."
Leading up to this game, though, the Pacers had been in all of 18 playoff games. Examining small samples will easily produce varying results. But that doesn't mean the results actually represent anything.
To see if missing the first shot actually does predict shooting percentage in the playoffs, I gathered a larger sample and tested the effect on the 53 players who made more than 100 field-goal attempts in the playoffs. Field-goal attempts were sorted independent of a player's first shot in each ball game. While the first shot determined which category a game's stats would fall under (FG% after making first shot, FG% after missing first shot), the first shot itself was subtracted from a player's shooting performance.
When players made their first shot, they went, on average, 44.8 percent from the field. When players missed it, they shot 44.4 percent. As if it weren't obvious already, a t-test showed no significant difference (p=0.35). Even when all shots were aggregated together (as opposed to aggregating player averages and treating each player in the sample the same), shooting percentage was not significantly different as players shot 46 percent (1,179-2,561) after making their first shot, and 47 percent (1,381-2,934) after missing.
The histogram below breaks this down for all 53 players. Twenty-five (to the left) had a worse shooting percentage if they missed the first shot, while 28 saw their shooting percentage improve:
Given the small sample size of the playoffs, some players will naturally end up in the tails of the distribution. Reggie Jackson's shooting dropped 0.297 percentage points if he missed his first shot; Boris Diaw's went up by 0.213 percentage points. An announcer might grab that stat and generalize from it—"Jackson just looks so much more confident after he makes his first shot. You can see it in his body language …"—but if Diaw and Jackson make the playoffs again next year, there's no real reason to think they'll continue to be such outliers.