Quantifying the Possible Effects of the Cubs’ Day-Game-Only Policy on Pitcher Durability Alan Reifman, Texas Tech University RESEARCH QUESTION: For 40 years ( ), Chicago’s Wrigley Field was the only MLB stadium without lights. The Cubs were thus the only team in these years to play all afternoon home games, creating uncomfortably hot and humid conditions for many, if not most, games. Baseball observers have long debated whether Wrigley’s day-game-only policy wore down Cub players, especially in 1969 when they faltered in the NL East title race against the New York Mets. Grochowski (2013, Chicago Sun-Times) found that from the Cubs won.443 of their September- October games, compared to playing.478 baseball overall, suggesting a possible heat-induced swoon. After lights were installed, however, the Cubs’ cumulative September-October record was indistinguishable from their overall mark. Because pitchers (a) are crucial to teams’ success, (b) are highly susceptible to fatigue, and (c) produce extensive data on durability (innings pitched) and effectiveness (e.g., earned runs allowed), the present study asks whether there is evidence for heat-related fatigue-effects on Cub pitchers, compared to pitchers for other teams. DATA: The study used data from Baseball Reference ( covering and Using players whose MLB careers began in 1960 or later allowed for a sample size of 141 player-seasons and, in the author’s judgment, kept players’ careers within a reasonable time span. Because managers’ decisions on removing pitchers likely differ in the two leagues due to the designated hitter, only NL seasons are included. The 1988 season was excluded because the first Wrigley night game did not occur until August 9 of that year, so 1988 could not be deemed purely a “lights” or “no lights” year. No years beyond 1996 were used, because interleague play began in 1997, thus introducing the DH into NL teams’ games for the first time. The strike-shortened 1981 and 1994 seasons were also excluded. ANALYTIC STRATEGY: For each pitcher in each season, the study examined trend-lines of pitching performance by month, as wear-and-tear (especially pitching for the day-game-era Cubs) would likely lead to progressive declines in performance from April to September-October. Two pitching metrics – innings per start and earned-runs per inning – were of primary interest. Comparing all Cub and non-Cub pitchers would have the problem that Cub and non-Cub pitchers likely differed on underlying physical abilities and not just the home park in which they pitched. For example, the Cubs might have put greater weight than other teams on endurance in drafting pitchers. To minimize physical differences between Cub and non-Cub pitchers, therefore, only pitchers who completed at least one full season as a Cubs starter and at least one full season starting for another NL club were studied. In other words, the identities of the players who pitched for the Cubs and for opposing teams were the same; they just pitched for the Cubs and for other teams in different years (see chart at right). All starts, at Wrigley and away, were studied, as even while pitching an evening road game, pitching day games at Wrigley could still take a cumulative toll. Of necessity, a player will be older when pitching for the Cubs than for another team (or vice-versa), so all analyses controlled statistically for pitcher age. The primary analyses focused on the years , producing samples of 53 player-seasons for the Cubs and 67 KEY FINDINGS: Pitchers averaged more starts per season for the Cubs (29.6) than for other NL teams (27.3), regardless of era (pre-lights/ or post-lights/ ). Heat-related fatigue-effects on day-game-only (pre-1988) Cub pitchers were modest. Perhaps as a Chicago managerial strategy to minimize pitcher burn-out, starters from averaged slightly fewer innings/start (roughly 6⅓) from May-August as a Cub than pitching for other teams (Figure 1). Pitchers in the day-game-only years also allowed slightly more runs per inning in May and August as a Cub than as a member of other NL teams (Figure 2). Because of the much larger number of innings a pitcher would have worked by August than by May, heat-induced fatigue would seem to be a more plausible explanation for performance decrements in August than in May. However, in September of the pre-lighting years, Cub pitchers were actually slightly more effective than those for other teams. player-seasons for other NL teams. Smaller samples of player-seasons from (9 for the Cubs, 12 for other teams) were used in supplementary analyses, in which no differences in pitching performance for the Cubs and for other NL teams would be expected. To pinpoint further the possible effect of day-game heat, a comparison of post-lights ( ) pitchers’ Cub and non-Cub statistics was undertaken. The argument for heat-related fatigue would be strengthened if monthly pitching decrements shown by pre-lights ( ) Cub pitchers were reduced after the installation of lights. Two findings with innings pitched/start supported this proposition (Cub pitchers threw.45 fewer innings per start in May than other NL hurlers pre-lights, but Cub pitchers’ May decrement shrunk to.13 post-lights; also Cub pitchers’ August innings/start shifted from a decrement of.25 pre-lights to essentially no different post-lights). However, there were also months in which Cub pitchers performed worse relative to their NL counterparts after the introduction of lights than before, failing to support the heat-effects hypothesis. Analyses of ER/inning showed similar results: Cub pitchers’ pre-lighting decrements improved post-lights for May and August, but not for other months. Photo by Author CONCLUSION: Adverse effects on Cub pitchers of playing during Wrigley Field’s day-game-only years seem to have been rare and, where they occurred, small in magnitude. Figure 1Figure 2