For a while I have been interested to find out how teams' regular season record correlates with their playoff performance. My main motivation was a recent experience being pleasantly surprised with the Spurs during regular seasons and shockingly dissatisfied with their playoff performances. Before jumping to any conclusions I wanted to start from evaluating what a reasonable expectations should be. For that purpose I looked at the records of NBA teams both in regular seasons and postseasons:

Regular season performance criterium is pretty straightforward - it is a win percentage, or rather fraction (decimal number between 0 and 1), which is often cited and at the end of season defines the seed order for the playoffs. The playoffs themselves are more tricky, because number of games for different teams is different, and number of series won is more important than percentage of the victories. I introduced the Playoff Success Coefficient as PSC = 0.25*SW + WF, where SW is a number of series the team won and WP is the percentage of victories in games. The 0.25 coefficient assures that the team winning more series would have higher PSC (the lowest WF for a winning team is 4/7 = 0.57, while the highest possible WF for a losing team goes from 3/7 in the first round to 7/11, to 11/15 and finally to 15/19 = 0.79 in the Finals). 0.25 also gives a series bonus = 1 to the Champion, which is kind of neat. I selected all teams that have been in the playoffs at least 3 times in last 5 seasons, and calculated RSWF (Regular Season Win Fraction) and PSC for their 3 most recent playoff appearances. Finally, I plotted RSWF versus PSC and got the following plot:

There is an evident correlation between team's success in the regular season and postseason. The average linear regression is shown with the thick green line. The teams above the line did better than the average in regular seasons, the teams below the line did better than the average in the postseason, and the team on the line (the Lakers) did just as expected. Most of the West is above the line, because they play against tougher competition in the postseason. Most of the East is below the line because they didn't need to win so many games to get into the playoffs. An amazing extreme example is the 76s, who have been in the playoffs three times while having combined regular season record below 0.5 (for comparison, the lowest Western team, NOH, needed to win 61.4% of their regular season games to get into the playoffs three times, just impressive how much easier it is to get to the playoffs from the East). It is evident also from the plot, that San Antonio extremely underachieved in the last three playoffs. Based on their regular season record, they should have had about as much of a playoff success as Dallas or Orlando. Hopefully, they will regress back to the mean in this spring. For now however the SPURS team is the SPort's Ultimate Regular Season team.