Example 12. Regression with Dummy Variable Regressors: Turkey Data

The data consists of weights (in lbs.) and the ages (in weeks) of 13 Thanksgiving turkeys. Of these 13 birds, four were reared in Georgia (G), four in Virginia (V) and five in Wisconsin (W). Let Y denote the weight, X denote the age, and O denote the origin of these turkey. The data is shown in the table below.

Source: Draper, N.R. and Smith, H. (1998). Applied Regression Analysis. John Wiley & Sons: New York.

Table 12: Turkey Data

 X      Y   O
28   13.3   G
20    8.9   G
32   15.1   G
22   10.4   G
29   13.1   V
27   12.4   V
28   13.2   V
26   11.8   V
21   11.5   W
27   14.2   W
29   15.4   W
23   13.1   W
25   13.8   W

Questions:

1. We seek to verify a linear relationship between weights and ages. How do we incorporate the effect of different origins into the model?
   
   
2. How is a regression set up and interpreted in such cases?
   
   
3. How can we compare whether the effect of age on weight is the same for each origin?

Keywords: Indicator variables, sets of regression lines, parallel regression lines