Has the number of home runs hit by major league teams been changing over time? For the 41 years from 1960 to 2000, the average number of home runs hit per game per team for each season was computed in order to assess any change over time. Initially, simple linear regression was used to study the trend in home runs hit over the period 1960 to 2000 by using year to predict the average number of home runs per game per team in that year. However, it was pointed out that after the 1976 season the manufacturer of major league baseballs was changed from Spaulding to Rawlings. Because the change in the baseball used might affect the number of home runs (for example, if Rawlings produces a livelier ball, this would likely lead to more home runs), it was decided to include an additional variable, namely

A multiple regression analysis was performed using the model:

_{Avg. home runs per game per team}=₀+₁(Year) +₂(Manufacturer)

The following results were obtained:

Analysis of Variance

SourcedfSum of SquaresModel (Regression)20.744384Error382.539330Parameter Estimates

VariabledfParameter EstimateStandard ErrorIntercept1-27.9118012.8900Year10.0149770.00650Manufacturer1-0.1078160.11573

Reference: Ref 11-2

The value ofR²is:

A.0.067.B.0.227.C.0.259.D.0.744.