Annual AL Hitting In Innings 7-9 Relative to Innings 1-6, 1930-2017

All data from the Baseball Reference Play Index. Perhaps this gives us some idea of the impact of relief pitching over time.

For each year I found the OPS, OBP and ISO for the entire league in innings 7-9 and divided that by the corresponding numbers for innings 1-6 (OPSRatio, OBPRatio, ISORatio). Time line charts for each are below. The slope of the trend line for each one is

OPS) -.001
OBP) -.0006
ISO) -.0018

So maybe the strongest downward trend is for ISO.

Then I divided the 88 year period into eleven 8 year periods (I know, that is arbitrary). The table below shows each ratio for each 8 year period and again, it seems like ISO is the one that has dropped off the most. For example, from 1930-37, ISO in innings 7-9 was 3.2% higher than in innings 1-6. But from 2010-2017, it was 11.7% lower.

Anyway, all three stats have generally been in relative decline in the late innings.

Period	OPSRatio	OBPRatio	ISORatio
1930-1937	1.016	1.015	1.032
1938-1945	1.009	1.020	0.982
1946-1953	1.008	1.013	0.994
1954-1961	1.006	1.010	0.993
1962-1969	0.997	1.007	0.972
1970-1977	0.998	1.010	0.965
1978-1985	0.984	0.999	0.950
1986-1993	0.968	0.987	0.920
1994-2001	0.955	0.979	0.906
2002-2009	0.938	0.973	0.877
2010-2017	0.938	0.970	0.883

Here are the correlations between the % of batters faced by relievers and the late to early hitting ratios

OBPRatio) -.686
OPSRatio) -.771   
ISORatio) -.713

So, as the % of batter faced by relievers increases, the OPS in innings 7-9 relative to innings 1-6 falls. The relationship is strong since the correlation is -.771. Squaring that (the r-squared) gets about .59, meaning that 59% of the variation across years in OPSratio is explained by changes in the % of batters faced by relievers. The more that relievers appear, the poorer the hitters do in late innings.

Here are the timeline charts