Since this result goes a bit against general intuition, it would make some sense to investigate it, which is what I did, though on a somewhat smaller scale than that undertaken by Manley.
Additional findings: A friend suggested that perhaps the home court advantage in the deciding game of a playoff series is reduced, so I checked that. Since 1990, the home team has gone 23-6 in the deciding game of playoff series (excluding this season). They should have gone 22.4-6.6. So, once again regular season records very well determine how strong the playoff home court advantage is.
So how do we account for this bias? My approach was to predict the winning percentage of home teams in each playoff series, then to compare this predicted winning percentage with the actual home court winning percentage. If the predicted winning percentage based on regular season stats was higher than the actual winning percentage of home teams in the playoffs, then we've found evidence that the home court advantage is muted in the playoffs. However, as mentioned above, I found very little difference between the two numbers. Note that my study uses playoff series back through 1990; I plan to increase the database, but I doubt seriously that the conclusions will change. Even if the numbers changed, it is hard to draw a conclusion about today's game based on data from many years ago when the game was different.
P(A beats B at A's Home) = (A's Hm Rec)(1-B's Rd Rec)(Lg Rd Rec) ----------------------------------------------------------------------------- [(A's Hm Rec)(1-B's Rd Rec)(Lg Rd Rec)+(1-A's Hm Rec)(B's Rd Rec)(Lg Hm Rec) A's Hm Rec Team A's Record at Home B's Rd Rec Team B's Record on the Road Lg Rd Rec The League Average Winning % on the Road Lg Hm Rec The League Average Winning % at Home
As an example, consider the Los Angeles Laker-San Antonio Spur playoff series of 1995. The Lakers won 70.7% of their home games in 1995 and 46.3% of their road games. The Spurs won 80.5% of their home games and 70.7% of their road games. Using the above equation, we find that the Lakers chances of winning at home were 37.4%, whereas the Spurs' chances of winning at home were 74.1%. The teams played three games on each others' court; the Lakers were expected to win 0.374*3 = 1.1 games at home and the Spurs were expected to win 0.741*3=2.2 games at home. In reality, the Lakers did win one game at home and the Spurs won two games at home, just as would be expected. Doing this for all the playoff series since 1990 (including 1996), I found that home teams were expected to win 67.6% of their games and actually won only 66.9%, a very small difference. (Strictly, this "prediction" is only a prediction given the number of games played in a series. A more true "prediction" would only use records and playoff seeding to estimate the number of home wins and losses, but that would be a little more work than I have time for.)
As a final technical note, I should mention that I used a global regular season league home court winning percentage of 62.6% in the calculations. Using the league home court winning percentage of each season individually changes the results by less than 1%.
Season Lg. Home Road Home Road Underdog Favorite Underdog Favorite PredictedPredictedActual Actual
Season Home % Underdog Win% Win% Favorite Win% Win% Home % Home % Hm Gms Hm Gms Hm W's Hm W% Hm W's Hm W%
95 0.597 BOS 0.488 0.439 ORL 0.951 0.439 0.421 0.937 2 2 2.7 0.679 1 0.250
95 0.597 ATL 0.585 0.366 IND 0.805 0.463 0.494 0.810 1 2 2.1 0.705 2 0.667
95 0.597 CLV 0.634 0.415 NYK 0.707 0.634 0.374 0.671 2 2 2.1 0.522 1 0.250
95 0.597 CHI 0.683 0.463 CHA 0.707 0.512 0.551 0.626 2 2 2.4 0.588 3 0.750
95 0.597 DEN 0.561 0.439 SAS 0.805 0.707 0.240 0.759 1 2 1.8 0.586 2 0.667
95 0.597 POR 0.634 0.439 PHO 0.780 0.659 0.349 0.731 1 2 1.8 0.604 2 0.667
95 0.597 HOU 0.610 0.537 UTA 0.805 0.659 0.326 0.680 2 3 2.7 0.539 2 0.400
95 0.597 LAL 0.707 0.463 SEA 0.780 0.610 0.480 0.711 2 2 2.4 0.596 3 0.750
95 0.597 IND 0.805 0.463 NYK 0.707 0.634 0.587 0.626 3 4 4.3 0.609 4 0.571
95 0.597 CHI 0.683 0.463 ORL 0.951 0.439 0.622 0.931 3 3 4.7 0.776 3 0.500
95 0.597 IND 0.805 0.463 ORL 0.951 0.439 0.759 0.931 3 4 6.0 0.857 7 1.000
95 0.597 LAL 0.707 0.463 SAS 0.805 0.707 0.374 0.741 3 3 3.3 0.557 3 0.500
95 0.597 HOU 0.610 0.537 PHO 0.780 0.659 0.326 0.647 3 4 3.6 0.510 4 0.571
95 0.597 HOU 0.610 0.537 SAS 0.805 0.707 0.279 0.680 3 3 2.9 0.479 1 0.167
95 0.597 HOU 0.610 0.537 ORL 0.951 0.439 0.544 0.910 2 2 2.9 0.727 2 0.500 33 40 45.5 0.624 40 0.548
94 0.612 DEN 0.683 0.341 SEA 0.902 0.634 0.426 0.914 2 3 3.6 0.719 2 0.400
94 0.612 POR 0.732 0.415 HOU 0.854 0.561 0.560 0.831 2 2 2.8 0.696 3 0.750
94 0.612 GSW 0.707 0.512 PHO 0.878 0.488 0.603 0.804 1 2 2.2 0.737 2 0.667
94 0.612 UTA 0.805 0.488 SAS 0.780 0.561 0.659 0.690 2 2 2.7 0.674 3 0.750
94 0.612 MIA 0.537 0.488 ATL 0.878 0.512 0.397 0.819 2 3 3.3 0.650 3 0.600
94 0.612 NJN 0.707 0.390 NYK 0.780 0.610 0.480 0.768 2 2 2.5 0.624 3 0.750
94 0.612 CLV 0.756 0.390 CHI 0.756 0.585 0.567 0.743 1 2 2.1 0.685 2 0.667
94 0.612 IND 0.707 0.439 ORL 0.756 0.463 0.626 0.703 1 2 2.0 0.677 2 0.667
94 0.612 CHI 0.756 0.585 NYK 0.780 0.610 0.542 0.601 3 4 4.0 0.576 7 1.000
94 0.612 IND 0.707 0.439 ATL 0.878 0.512 0.579 0.846 3 3 4.3 0.713 5 0.833
94 0.612 IND 0.707 0.439 NYK 0.780 0.610 0.480 0.731 3 4 4.4 0.623 5 0.714
94 0.612 PHO 0.878 0.488 HOU 0.854 0.561 0.771 0.785 3 4 5.5 0.779 3 0.429
94 0.612 DEN 0.683 0.341 UTA 0.805 0.488 0.575 0.826 3 4 5.0 0.718 5 0.714
94 0.612 UTA 0.805 0.488 HOU 0.854 0.561 0.659 0.785 2 3 3.7 0.735 4 0.800
94 0.612 NYK 0.780 0.610 HOU 0.854 0.561 0.624 0.690 3 4 4.6 0.662 5 0.714 66 84 98.1 0.654 94 0.627
93 0.611 IND 0.659 0.341 NYK 0.902 0.561 0.474 0.914 2 2 2.8 0.694 3 0.750
93 0.611 ATL 0.610 0.439 CHI 0.756 0.634 0.350 0.703 1 2 1.8 0.585 2 0.667
93 0.611 NJN 0.634 0.415 CLV 0.854 0.463 0.545 0.831 2 3 3.6 0.717 3 0.600
93 0.611 CHA 0.537 0.537 BOS 0.683 0.488 0.421 0.526 2 2 1.9 0.474 3 0.750
93 0.611 LAL 0.488 0.463 PHO 0.854 0.659 0.228 0.801 2 3 2.9 0.572 1 0.200
93 0.611 LAC 0.659 0.341 HOU 0.756 0.585 0.449 0.781 2 3 3.2 0.649 3 0.600
93 0.611 UTA 0.683 0.463 SEA 0.805 0.537 0.526 0.741 2 3 3.3 0.655 3 0.600
93 0.611 SAS 0.756 0.439 POR 0.732 0.512 0.638 0.676 2 2 2.6 0.657 3 0.750
93 0.611 CHA 0.537 0.537 NYK 0.902 0.561 0.351 0.827 2 3 3.2 0.637 4 0.800
93 0.611 CLV 0.854 0.463 CHI 0.756 0.634 0.668 0.682 2 2 2.7 0.675 2 0.500
93 0.611 CHI 0.756 0.634 NYK 0.902 0.561 0.592 0.761 3 3 4.1 0.676 5 0.833
93 0.611 SAS 0.756 0.439 PHO 0.854 0.659 0.490 0.817 3 3 3.9 0.653 5 0.833
93 0.611 HOU 0.756 0.585 SEA 0.805 0.537 0.615 0.636 3 4 4.4 0.627 7 1.000
93 0.611 SEA 0.805 0.537 PHO 0.854 0.659 0.561 0.751 3 4 4.7 0.669 5 0.714
93 0.611 CHI 0.756 0.634 PHO 0.854 0.659 0.490 0.668 3 3 3.5 0.579 1 0.167 100 126 146.5 0.648 144 0.637
92 0.631 MIA 0.683 0.244 CHI 0.878 0.756 0.293 0.930 1 2 2.2 0.718 2 0.667
92 0.631 IND 0.634 0.341 BOS 0.829 0.415 0.594 0.848 1 2 2.3 0.764 2 0.667
92 0.631 NJN 0.610 0.366 CLV 0.854 0.537 0.446 0.858 2 2 2.6 0.652 3 0.750
92 0.631 DET 0.610 0.561 NYK 0.732 0.512 0.471 0.560 2 3 2.6 0.525 3 0.600
92 0.631 LAL 0.585 0.463 POR 0.805 0.585 0.374 0.741 2 2 2.2 0.557 3 0.750
92 0.631 LAC 0.707 0.390 UTA 0.902 0.439 0.648 0.896 2 3 4.0 0.797 5 1.000
92 0.631 SEA 0.683 0.463 GSW 0.756 0.585 0.477 0.682 2 2 2.3 0.579 3 0.750
92 0.631 SAS 0.756 0.390 PHO 0.878 0.415 0.723 0.870 1 2 2.5 0.821 2 0.667
92 0.631 BOS 0.829 0.415 CLV 0.854 0.537 0.715 0.831 3 4 5.5 0.781 5 0.714
92 0.631 NYK 0.732 0.512 CHI 0.878 0.756 0.345 0.804 3 4 4.2 0.607 5 0.714
92 0.631 CLV 0.854 0.537 CHI 0.878 0.756 0.529 0.788 3 3 4.0 0.659 3 0.500
92 0.631 PHO 0.878 0.415 POR 0.805 0.585 0.753 0.777 2 3 3.8 0.767 4 0.800
92 0.631 SEA 0.683 0.463 UTA 0.902 0.439 0.622 0.865 2 3 3.8 0.768 4 0.800
92 0.631 UTA 0.902 0.439 POR 0.805 0.585 0.797 0.759 3 3 4.7 0.778 5 0.833
92 0.631 POR 0.805 0.585 CHI 0.878 0.756 0.443 0.753 3 3 3.6 0.598 3 0.500 132 167 196.8 0.658 196 0.656
91 0.659 NYK 0.512 0.439 CHI 0.854 0.634 0.266 0.817 1 2 1.9 0.633 2 0.667
91 0.659 IND 0.707 0.293 BOS 0.854 0.512 0.579 0.894 2 3 3.8 0.768 3 0.600
91 0.659 ATL 0.707 0.341 DET 0.780 0.439 0.648 0.804 2 3 3.7 0.742 3 0.600
91 0.659 PHI 0.707 0.366 MIL 0.805 0.366 0.714 0.810 1 2 2.3 0.778 1 0.333
91 0.659 SEA 0.683 0.317 POR 0.878 0.659 0.400 0.903 2 3 3.5 0.702 5 1.000
91 0.659 GSW 0.732 0.341 SAS 0.805 0.537 0.585 0.826 2 2 2.8 0.705 3 0.750
91 0.659 HOU 0.756 0.512 LAL 0.805 0.610 0.542 0.701 1 2 1.9 0.648 2 0.667
91 0.659 UTA 0.878 0.439 PHO 0.780 0.561 0.771 0.731 2 2 3.0 0.751 3 0.750
91 0.659 PHI 0.707 0.366 CHI 0.854 0.634 0.454 0.858 2 3 3.5 0.697 4 0.800
91 0.659 DET 0.780 0.439 BOS 0.854 0.512 0.669 0.817 3 3 4.5 0.743 3 0.500
91 0.659 DET 0.780 0.439 CHI 0.854 0.634 0.551 0.817 2 2 2.7 0.684 2 0.500
91 0.659 GSW 0.732 0.341 LAL 0.805 0.610 0.510 0.826 2 3 3.5 0.700 2 0.400
91 0.659 UTA 0.878 0.439 POR 0.878 0.659 0.690 0.846 2 3 3.9 0.784 4 0.800
91 0.659 LAL 0.805 0.610 POR 0.878 0.659 0.561 0.734 3 3 3.9 0.647 5 0.833
91 0.659 LAL 0.805 0.610 CHI 0.854 0.634 0.587 0.690 3 2 3.1 0.628 1 0.200 162 205 245.0 0.668 239 0.651
90 0.644 NYK 0.707 0.390 BOS 0.732 0.537 0.555 0.718 2 3 3.3 0.653 4 0.800
90 0.644 IND 0.683 0.341 DET 0.854 0.585 0.477 0.870 1 2 2.2 0.739 2 0.667
90 0.644 CLV 0.659 0.366 PHI 0.829 0.463 0.572 0.834 2 3 3.6 0.729 5 1.000
90 0.644 MIL 0.659 0.415 CHI 0.878 0.463 0.572 0.859 2 2 2.9 0.715 3 0.750
90 0.644 HOU 0.756 0.244 LAL 0.902 0.634 0.517 0.945 2 2 2.9 0.731 3 0.750
90 0.644 PHO 0.780 0.537 UTA 0.878 0.463 0.711 0.788 2 3 3.8 0.757 2 0.400
90 0.644 DEN 0.683 0.366 SAS 0.829 0.537 0.526 0.834 1 2 2.2 0.732 2 0.667
90 0.644 DAL 0.732 0.415 POR 0.854 0.585 0.536 0.831 1 2 2.2 0.733 2 0.667
90 0.644 PHI 0.829 0.463 CHI 0.878 0.463 0.771 0.833 2 3 4.0 0.808 4 0.800
90 0.644 NYK 0.707 0.390 DET 0.854 0.585 0.506 0.845 2 3 3.5 0.709 4 0.800
90 0.644 CHI 0.878 0.463 DET 0.854 0.585 0.753 0.801 3 4 5.5 0.781 7 1.000
90 0.644 SAS 0.829 0.537 POR 0.854 0.585 0.673 0.751 3 4 5.0 0.717 7 1.000
90 0.644 PHO 0.780 0.537 LAL 0.902 0.634 0.551 0.827 2 3 3.6 0.716 3 0.600
90 0.644 PHO 0.780 0.537 POR 0.854 0.585 0.601 0.751 3 3 4.1 0.676 5 0.833
90 0.644 POR 0.854 0.585 DET 0.854 0.585 0.712 0.712 3 2 3.6 0.712 1 0.200 193 246 297.3 0.677 293 0.667
96 0.604 MIA 0.634 0.390 CHI 0.951 0.805 0.201 0.948 1 2 2.1 0.699 2 0.667
96 0.604 NYK 0.634 0.512 CLV 0.634 0.512 0.497 0.497 1 2 1.5 0.497 1 0.333
96 0.604 DET 0.732 0.390 ORL 0.902 0.561 0.560 0.896 1 2 2.4 0.784 2 0.667
96 0.604 ATL 0.634 0.488 IND 0.780 0.488 0.521 0.690 2 3 3.1 0.623 2 0.400
96 0.604 SAC 0.634 0.317 SEA 0.927 0.634 0.374 0.942 2 2 2.6 0.658 1 0.250
96 0.604 HOU 0.659 0.512 LAL 0.732 0.561 0.474 0.608 2 2 2.2 0.541 3 0.750
96 0.604 PHO 0.610 0.390 SAS 0.805 0.634 0.350 0.794 2 2 2.3 0.572 3 0.750
96 0.604 POR 0.634 0.439 UTA 0.829 0.512 0.497 0.788 2 3 3.4 0.671 5 1.000
96 0.604 NYK 0.634 0.512 CHI 0.951 0.805 0.201 0.917 2 3 3.2 0.631 4 0.800
96 0.604 ATL 0.634 0.488 ORL 0.902 0.561 0.448 0.853 2 3 3.5 0.691 4 0.800
96 0.604 ORL 0.902 0.561 CHI 0.951 0.805 0.573 0.901 2 2 2.9 0.737 2 0.500
96 0.604 HOU 0.659 0.512 SEA 0.927 0.634 0.399 0.878 2 2 2.6 0.639 2 0.500
96 0.604 UTA 0.829 0.512 SAS 0.805 0.634 0.626 0.701 3 3 4.0 0.664 5 0.833
96 0.604 UTA 0.829 0.512 SEA 0.927 0.634 0.626 0.878 3 4 5.4 0.770 5 0.714
96 0.604 SEA 0.927 0.634 CHI 0.951 0.805 0.647 0.870 3 3 4.6 0.759 5 0.833
223 284 342.7 0.676 339 0.669
193 246 297.3 0.677 293 0.667