diff --git a/scripts/Chap8.trajectories.R b/scripts/Chap8.trajectories.R
index 0d1d203..5a87263 100644
--- a/scripts/Chap8.trajectories.R
+++ b/scripts/Chap8.trajectories.R
@@ -33,7 +33,7 @@ get.stats <- function(player.id){
   d$Age <- d$yearID - byear
   d$SLG <- with(d, (H - X2B - X3B - HR +
                     2 * X2B + 3 * X3B + 4 * HR) / AB)
-  d$OBP <- with(d, (H + BB) / (H + AB + BB + SF))
+  d$OBP <- with(d, (H + BB + HBP) / (AB + BB + SF + HBP)) # HBP are part of OBP formula, H should not be in denominator.
   d$OPS <- with(d, SLG + OBP)
   d
 }
@@ -298,4 +298,4 @@ ggplot(dnew, aes(Age, OPS)) + geom_point(size=4) +
   stat_smooth(method="lm", se=FALSE, size=1.5,
               formula=y ~ poly(x, 2, raw=TRUE))  + theme_bw()
 
-################################################################
\ No newline at end of file
+################################################################