Statistical modelling/Milk offtake

GenStat also produces warning messages as appropriate during the analysis. The messages shown here, which should be read in conjunction with the scatter diagram on the previous page, were produced when non-parallel lines were fitted.

* MESSAGE: The following units have large standardized residuals:
Unit Response Residual
39 5.600 5.96
40 4.400 3.79

* MESSAGE: The error variance does not appear to be constant:
large responses are more variable than small responses

* MESSAGE: The following units have high leverage:

Unit Response Leverage
65 1.300 0.068
67 1.600 0.068
80 1.700 0.068
85 2.300 0.068
etc

Message 1. The two units have standardised residuals (calculated as the deviation of an observation from its fitted value divided by the overall residual standard deviation) meaning that they fall some distance away from the fitted line and are ´outliers´.

Message 2. This suggests that the assumption the y-variable has a constant variance may not be tenable.

Message 3. Units with high leverage are those points that have a strong influence on the direction of the regression line. These points in this example are those that lie to the extreme right for calves aged 12 months and beyond.