NBA Moneyball
Narcel Reedus - Data Analyst
October 12, 2017
Summary
NBA Moneyball uses a multiple linear regression of NBA stats and previous Win/Loss team records to predict the chances of NBA teams reaching the playoffs. The term Moneyball was made popular by the 2011 movie starring Brad Pit as Billy Beane, the coach of the flailing Oakland A’s, who used a data analytics approach to winning ball games.
In this regression Model4 has an .8127 accuracy.
Load libraries into RStudio
library(dplyr)
library(magrittr)
library(ggplot2)Load NBA training data with read.csv
NBA_train <- read.csv("C:/Users/narce/OneDrive/Documents/GitHub/NBA/NBA/NBA_train.csv")Assign the NBA_train data to NBA
NBA <-NBA_trainExamine the structure of the data (835 observations and 20 variables) with the str function.
str(NBA)## 'data.frame': 835 obs. of 20 variables:
## $ SeasonEnd: int 1980 1980 1980 1980 1980 1980 1980 1980 1980 1980 ...
## $ Team : Factor w/ 37 levels "Atlanta Hawks",..: 1 2 5 6 8 9 10 11 12 13 ...
## $ Playoffs : int 1 1 0 0 0 0 0 1 0 1 ...
## $ W : int 50 61 30 37 30 16 24 41 37 47 ...
## $ PTS : int 8573 9303 8813 9360 8878 8933 8493 9084 9119 8860 ...
## $ oppPTS : int 8334 8664 9035 9332 9240 9609 8853 9070 9176 8603 ...
## $ FG : int 3261 3617 3362 3811 3462 3643 3527 3599 3639 3582 ...
## $ FGA : int 7027 7387 6943 8041 7470 7596 7318 7496 7689 7489 ...
## $ X2P : int 3248 3455 3292 3775 3379 3586 3500 3495 3551 3557 ...
## $ X2PA : int 6952 6965 6668 7854 7215 7377 7197 7117 7375 7375 ...
## $ X3P : int 13 162 70 36 83 57 27 104 88 25 ...
## $ X3PA : int 75 422 275 187 255 219 121 379 314 114 ...
## $ FT : int 2038 1907 2019 1702 1871 1590 1412 1782 1753 1671 ...
## $ FTA : int 2645 2449 2592 2205 2539 2149 1914 2326 2333 2250 ...
## $ ORB : int 1369 1227 1115 1307 1311 1226 1155 1394 1398 1187 ...
## $ DRB : int 2406 2457 2465 2381 2524 2415 2437 2217 2326 2429 ...
## $ AST : int 1913 2198 2152 2108 2079 1950 2028 2149 2148 2123 ...
## $ STL : int 782 809 704 764 746 783 779 782 900 863 ...
## $ BLK : int 539 308 392 342 404 562 339 373 530 356 ...
## $ TOV : int 1495 1539 1684 1370 1533 1742 1492 1565 1517 1439 ...Below is a list of variables definitions
SeasonEnd: Year the season ended
Team: Name of team
Playoffs: Binary variable for playoff appearance
W: Wins (regular season)
PTS: Points scored (regular season)
oppPTS:Oopponent points scored (regular season)
FG, FGA: Field goals (including three pointers)
X2P, X2PA: 2-pointers
X3P, X3PA: 3-pointers
FT, FTA: Free throws
ORB, DRB: Offensive and defensive rebounds
AST: Assists
STL: Steals
BLK: Blocks
TOV: Turnovers
The goal here is to determine how many regular season wins are needed for and NBA team to make it to the playoffs. I start by grouping the data by the number of wins and creating new features: number of playoff appearances and the percentage of playoff appearances fracPO divided by total number of wins.
tmp<- group_by(NBA, W) %>% summarise(nTot = n(), nPO = sum(Playoffs), fracPO = nPO/nTot)
View(tmp)Here I plot the number of Wins on the x axis and playoff appearance on the y axis. This plot shows that (45>) wins have a (90%) chance of making it to the playoffs.
plot(tmp$W, tmp$fracPO, pch = 21, col = "red2", bg = "orange",
xlab = "Wins", ylab = "Chance at Making the NBA Playoffs", main = "NBA Playoff Appearance / Regular Season Wins")
abline(h = 0.9, lty = 3, col = "red2")
abline(v = 35, lty = 3, col = "blue2")
abline(v = 45, lty = 3, col = "blue2")
Wins are predicted by calculating the difference between points scored (PTS) and points allowed (oppPTS)
NBA$PTSdiff <- NBA$PTS - NBA$oppPTSCheck the linear relationship between PTSdiff and Wins
plot(NBA$PTSdiff, NBA$W, pch = 21, col = "red2", bg = "orange",
xlab = "PTS Difference", ylab = "Wins", main = "Wins / Points Scored")
### This is the linear regression Model for Wins - PTSdiff = independent variable and W = dependent variable
Wins_Reg <- lm(W ~ PTSdiff, data = NBA)
summary(Wins_Reg)##
## Call:
## lm(formula = W ~ PTSdiff, data = NBA)
##
## Residuals:
## Min 1Q Median 3Q Max
## -9.7393 -2.1018 -0.0672 2.0265 10.6026
##
## Coefficients:
## Estimate Std. Error t value Pr(>|t|)
## (Intercept) 4.100e+01 1.059e-01 387.0 <2e-16 ***
## PTSdiff 3.259e-02 2.793e-04 116.7 <2e-16 ***
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Residual standard error: 3.061 on 833 degrees of freedom
## Multiple R-squared: 0.9423, Adjusted R-squared: 0.9423
## F-statistic: 1.361e+04 on 1 and 833 DF, p-value: < 2.2e-16W = 41 + 0.0325*(NBA$PTSdiff)The linear regression Model computes the PTSdiff needed to attain W=> 45, PTSdiff>122.8. This means a teams must score 122.8 more points by the end of the season to make it to the playoffs.
This formula will predict points scored. The dependent variable = PTS, independent variable = (X2PA, X3PA, FTA, ORB, DRB, AST, ST, BL, TOV)
PointsRS <- lm(PTS ~ X2PA + X3PA + FTA + AST + ORB + DRB + TOV + STL + BLK, data = NBA)
summary(PointsRS)##
## Call:
## lm(formula = PTS ~ X2PA + X3PA + FTA + AST + ORB + DRB + TOV +
## STL + BLK, data = NBA)
##
## Residuals:
## Min 1Q Median 3Q Max
## -527.40 -119.83 7.83 120.67 564.71
##
## Coefficients:
## Estimate Std. Error t value Pr(>|t|)
## (Intercept) -2.051e+03 2.035e+02 -10.078 <2e-16 ***
## X2PA 1.043e+00 2.957e-02 35.274 <2e-16 ***
## X3PA 1.259e+00 3.843e-02 32.747 <2e-16 ***
## FTA 1.128e+00 3.373e-02 33.440 <2e-16 ***
## AST 8.858e-01 4.396e-02 20.150 <2e-16 ***
## ORB -9.554e-01 7.792e-02 -12.261 <2e-16 ***
## DRB 3.883e-02 6.157e-02 0.631 0.5285
## TOV -2.475e-02 6.118e-02 -0.405 0.6859
## STL -1.992e-01 9.181e-02 -2.169 0.0303 *
## BLK -5.576e-02 8.782e-02 -0.635 0.5256
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Residual standard error: 185.5 on 825 degrees of freedom
## Multiple R-squared: 0.8992, Adjusted R-squared: 0.8981
## F-statistic: 817.3 on 9 and 825 DF, p-value: < 2.2e-16The summary shows a significant correlation between X2PA, X3PA, FTA, AST, ORB and Wins (each having 3 stars). Independent variables DRB, TOV, STL, BLK do not show a strong correlation to determining Win R2 value (0.8992) bus shows a good linear relationship between the independent and dependent variables to points scored.
Sum of Squared Errors
SSE <- sum(PointsRS$residuals^2)
print(SSE)## [1] 28394314Root Mean Error
RMSE <- sqrt(SSE/PointsRS$df.residual)
RMSE## [1] 185.5191mean(NBA$PTS)## [1] 8370.24Fractional error = 2.2%
Running Model #2 removing TOV
PointsRS2 <- lm(PTS ~ X2PA + X3PA + FTA + AST + ORB + DRB + STL + BLK, data = NBA)
summary(PointsRS2)##
## Call:
## lm(formula = PTS ~ X2PA + X3PA + FTA + AST + ORB + DRB + STL +
## BLK, data = NBA)
##
## Residuals:
## Min 1Q Median 3Q Max
## -526.79 -121.09 6.37 120.74 565.94
##
## Coefficients:
## Estimate Std. Error t value Pr(>|t|)
## (Intercept) -2.077e+03 1.931e+02 -10.755 <2e-16 ***
## X2PA 1.044e+00 2.951e-02 35.366 <2e-16 ***
## X3PA 1.263e+00 3.703e-02 34.099 <2e-16 ***
## FTA 1.125e+00 3.308e-02 34.023 <2e-16 ***
## AST 8.861e-01 4.393e-02 20.173 <2e-16 ***
## ORB -9.581e-01 7.758e-02 -12.350 <2e-16 ***
## DRB 3.892e-02 6.154e-02 0.632 0.5273
## STL -2.068e-01 8.984e-02 -2.301 0.0216 *
## BLK -5.863e-02 8.749e-02 -0.670 0.5029
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Residual standard error: 185.4 on 826 degrees of freedom
## Multiple R-squared: 0.8991, Adjusted R-squared: 0.8982
## F-statistic: 920.4 on 8 and 826 DF, p-value: < 2.2e-16By removing TOV the R2 = 0.8991 which is higher than Model1 at 0.8992. In Model3 DRB is removed.
PointsRS3 <- lm(PTS ~ X2PA + X3PA + FTA + AST + ORB + STL + BLK, data = NBA)
summary(PointsRS3)##
## Call:
## lm(formula = PTS ~ X2PA + X3PA + FTA + AST + ORB + STL + BLK,
## data = NBA)
##
## Residuals:
## Min 1Q Median 3Q Max
## -523.79 -121.64 6.07 120.81 573.64
##
## Coefficients:
## Estimate Std. Error t value Pr(>|t|)
## (Intercept) -2.015e+03 1.670e+02 -12.068 < 2e-16 ***
## X2PA 1.048e+00 2.852e-02 36.753 < 2e-16 ***
## X3PA 1.271e+00 3.475e-02 36.568 < 2e-16 ***
## FTA 1.128e+00 3.270e-02 34.506 < 2e-16 ***
## AST 8.909e-01 4.326e-02 20.597 < 2e-16 ***
## ORB -9.702e-01 7.519e-02 -12.903 < 2e-16 ***
## STL -2.276e-01 8.356e-02 -2.724 0.00659 **
## BLK -3.882e-02 8.165e-02 -0.475 0.63462
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Residual standard error: 185.4 on 827 degrees of freedom
## Multiple R-squared: 0.8991, Adjusted R-squared: 0.8982
## F-statistic: 1053 on 7 and 827 DF, p-value: < 2.2e-16The Model3 R2 is the same as Model1 at 0.8991. In Model4 BLK is removed.
PointsRS4 <- lm(PTS ~ X2PA + X3PA + FTA + AST + ORB + STL, data = NBA)
summary(PointsRS4)##
## Call:
## lm(formula = PTS ~ X2PA + X3PA + FTA + AST + ORB + STL, data = NBA)
##
## Residuals:
## Min 1Q Median 3Q Max
## -523.33 -122.02 6.93 120.68 568.26
##
## Coefficients:
## Estimate Std. Error t value Pr(>|t|)
## (Intercept) -2.033e+03 1.629e+02 -12.475 < 2e-16 ***
## X2PA 1.050e+00 2.829e-02 37.117 < 2e-16 ***
## X3PA 1.273e+00 3.441e-02 37.001 < 2e-16 ***
## FTA 1.127e+00 3.260e-02 34.581 < 2e-16 ***
## AST 8.884e-01 4.292e-02 20.701 < 2e-16 ***
## ORB -9.743e-01 7.465e-02 -13.051 < 2e-16 ***
## STL -2.268e-01 8.350e-02 -2.717 0.00673 **
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
##
## Residual standard error: 185.3 on 828 degrees of freedom
## Multiple R-squared: 0.8991, Adjusted R-squared: 0.8983
## F-statistic: 1229 on 6 and 828 DF, p-value: < 2.2e-16The Model4 R2 is the same as Model1 and Model3 at 0.8991. A chart provides a closer look at SSE and RMSE of Model4.
SSE_4 <-sum(PointsRS4$residuals^2)
RSME_4 <- sqrt(SSE_4/nrow(NBA))The values for Model4 (PointsRS4) are SSE = 28421464.9 and RMSE = 184.493 compared to the Model1 RMSE = 185.5191. They are essentially the same.
Time for predictions
New I load in the in NBA_test data.
NBA_test <- read.csv("C:/Users/narce/OneDrive/Documents/GitHub/NBA/NBA/NBA_test.csv")Attempting to predict using Model4, the number of points in the 2012-2013 season using the predict function and the new NBA_test.csv data.
PointsPredictions <- predict(PointsRS4, newdata = NBA_test)
summary(PointsPredictions)## Min. 1st Qu. Median Mean 3rd Qu. Max.
## 7507 7843 8005 8032 8169 8620To determine the accuracy of Model4 (0.8991) the SSE, SST, RMSE must be analyzed.
SSE <- sum((PointsPredictions - NBA_test$PTS)^2)
SSE## [1] 1079739SST <- sum((mean(NBA$PTS) - NBA_test$PTS)^2)R2 is calculated with 1 minus the sum of squared errors divided by the total sum of squares.
R2 <- 1 - SSE/SST
R2## [1] 0.8127142RMSE <- sqrt(SSE/nrow(NBA_test))
RMSE## [1] 196.3723