Comparing the Bradley Terry model to betting odds

In this previous post, I described the Bradley-Terry model and showed how we could use it to predict game outcomes in the NBA 2018-19 regular season. After ffitting the Bradley-Terry model on the first half of the regular season (with and without home advantage), I used the model to predict win probabilities for the second half of the season. The models gave test Brier scores of 0.213 and 0.219, which I said was no longer than random 50-50 guessing (which has a Brier score of 0.25).

In private correspondence, Justin Dyer pointed out to me that it’s not clear a priori that Brier scores of ~0.21 are bad: we should benchmark the scores against other models, or the Brier scores implied by bookmaking odds. Presumably bookmaking odds incorporate a lot more information than the simple Bradley-Terry model above (e.g. injuries, team’s recent form).

In this post, we compare the Bradley-Terry model against betting odds for that same data. All the code in this post can be found in one script here.

Getting the data

Betting odds data was obtained from here and games data was obtained from here.

First, import the packages that we will use:

library(DBI)
library(plotROC)
library(readr)
library(tidyverse)

Next, we pull out the relevant game data from the SQLite database:

seasonYear <- 2018  # represents 2018-2019 season

# this filepath specific to my local drive
mainFile <- "basketball.sqlite"

# get all regular season games (only relevant columns 
# selected)
mydb <- dbConnect(RSQLite::SQLite(), mainFile)
df <- dbGetQuery(mydb, "SELECT * FROM Game")
dbDisconnect(mydb)
season_df <- df %>% mutate(GAME_DATE = as.Date(GAME_DATE),
                           SEASON = as.numeric(SEASON)) %>% 
  filter(SEASON == seasonYear) %>%
  select(GAME_DATE, TEAM_NAME_HOME, TEAM_NAME_AWAY, WL_HOME) %>%
  arrange(GAME_DATE)

head(season_df)
#    GAME_DATE        TEAM_NAME_HOME         TEAM_NAME_AWAY WL_HOME
# 1 2018-10-16 Golden State Warriors  Oklahoma City Thunder       W
# 2 2018-10-16        Boston Celtics     Philadelphia 76ers       W
# 3 2018-10-17     San Antonio Spurs Minnesota Timberwolves       W
# 4 2018-10-17       New York Knicks          Atlanta Hawks       W
# 5 2018-10-17          Phoenix Suns       Dallas Mavericks       W
# 6 2018-10-17           LA Clippers         Denver Nuggets       L

After that, we pull out the relevant betting odds data. The data frame contains several different types of odds as well as odds from different bookmakers. We use the Average_Line_ML column, representing the average moneyline bet from the bookmakers in the dataset. In the last line, we convert the moneyline odds to win probability (see this website for more details).

# this filepath specific to my local drive
vegas <- read_csv("vegas.txt")

bet_df <- vegas %>% filter(Location == "home") %>%
  select(Date, Team, OppTeam, line = Average_Line_ML) %>%
  mutate(winProb = ifelse(line < 0, -line / (100 - line), 100 / (100 + line)))

head(bet_df)
# # A tibble: 6 × 5
#   Date       Team         OppTeam         line winProb
#   <date>     <chr>        <chr>          <dbl>   <dbl>
# 1 2018-10-16 Boston       Philadelphia   -210.   0.678
# 2 2018-10-16 Golden State Oklahoma City -1015.   0.910
# 3 2018-10-17 Charlotte    Milwaukee       145    0.408
# 4 2018-10-17 Detroit      Brooklyn       -242.   0.708
# 5 2018-10-17 Indiana      Memphis        -294.   0.746
# 6 2018-10-17 Orlando      Miami           116    0.463

Join the two datasets together:

season_teams <- sort(unique(season_df$TEAM_NAME_HOME))
bet_teams <- sort(unique(bet_df$Team))
names(bet_teams) <- season_teams

df <- season_df %>%
  transmute(Date = GAME_DATE,
            Team = bet_teams[TEAM_NAME_HOME],
            OppTeam = bet_teams[TEAM_NAME_AWAY],
            HomeWinLoss = WL_HOME) %>%
  left_join(bet_df)

For some reason, 4 of the games did not have betting odds data. We impute a win probability of 0.5 to them (i.e. no information, guess a 50-50 coin flip):

df$winProb[is.na(df$winProb)] <- 0.5

Fitting the Bradley-Terry model

The chunk of code below fits the Bradley-Terry model (with and without home advantage) on the training data. This is an abbreviated version of the code in the previous post; go there if you would like more detail.

# Get data in a form suitable for the BT model
get_data_vec <- function(home_team, away_team, teams) {
  vec <- rep(0, length(teams))
  vec[teams == home_team] <- 1
  vec[teams == away_team] <- -1
  vec
}

X <- apply(df, 1, 
           function(row) get_data_vec(row["Team"], 
                                      row["OppTeam"], 
                                      bet_teams))
X <- t(X)
colnames(X) <- bet_teams
dim(X)
y <- as.numeric(df$HomeWinLoss == "W")
bt_df <- as.data.frame(cbind(X, y))

# split into train and test
n_train <- nrow(X) / 2
train_df <- bt_df[1:n_train, ]
test_df <- bt_df[(n_train + 1):nrow(X), ]

# train BT models
train_mod_home <- glm(y ~ ., data = train_df, family = binomial())
train_mod_no_home <- glm(y ~ . + 0, data = train_df, family = binomial())

Comparing predictions

We get the predictions for the models and betting odds on the test set:

pred_home <- predict(train_mod_home, newdata = test_df, type = "response")
pred_no_home <- predict(train_mod_no_home, newdata = test_df, type = "response")
pred_bet <- df$winProb[(n_train + 1):nrow(X)]

Before comparing the models on test performance, let’s have a look at the predictions themselves. The code below plots the win probability predictions from betting odds against those from the Bradley-Terry model with home advantage.

plot(pred_home, pred_bet, pch = 16, cex = 0.5,
     xlab = "Prob. of home win (BT model w. home adv.)",
     ylab = "Prob. of home win (betting odds)")
abline(0, 1)

It looks like there is quite a bit of variation between the two but they generally give the same directional outcome (i.e. they would predict the same team winning).

Now for test performance. The code below plots the test ROC curves for the 3 models:

pred_df <- data.frame(
  truth = test_df$y,
  pred_home = pred_home,
  pred_no_home = pred_no_home,
  pred_bet = pred_bet
) %>%
  pivot_longer(pred_home:pred_bet, 
               names_to = "model", 
               values_to = "prediction")

roc_plot <- ggplot(pred_df) +
  geom_roc(aes(d = truth, m = prediction, color = model), labels = FALSE)

roc_plot +
  labs(x = "True positive fraction",
       y = "False positive fraction",
       title = "Test set ROC plots") +
  geom_abline(slope = 1, intercept = 0, linetype = "dashed") +
  theme(legend.position = "bottom")

From the ROC curve plot, betting odds (red line) clearly outperform the two Bradley-Terry models, but not by much! Finally, let’s compute the test AUCs and Brier scores:

# compute AUC
calc_auc(roc_plot)
#   PANEL group       AUC
# 1     1     1 0.7421478
# 2     1     2 0.7120678
# 3     1     3 0.7133042

# compute Brier score
mean((pred_bet - test_df$y)^2)
# [1] 0.2027192
mean((pred_home - test_df$y)^2)
# [1] 0.2130266
mean((pred_no_home - test_df$y)^2)
# [1] 0.2186777

Betting odds has a better test Brier score, but only marginally: 0.203 vs. 0.213 & 0.219! This is quite surprising in that the Bradley-Terrry model is so simple and uses much less information than betting odds, yet its performance is not too far off.

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