Combining the models for the two groups

The variable of interest can often be modelled by a normal distribution in each group with the same standard deviation, σ, but possibly with different means in the two groups.

These two models can be combined into a single general linear model, using two 'explanatory variables', di (1) and di (2), that can only take the values 0 or 1 and are called indicator variables.

Note that this GLM does not have a constant term (intercept).

Matrix representation of the model

The next diagram shows this general linear model in matrix form for a data set with 6 observations from each of 2 groups.

Click on any y-value to see how the indicator variables pick out the appropriate mean for its group.

Alternative parameterisation with baseline group

A different way to express the same model is more useful — it simplifies the test for equal group means and can be easily extended to more complex situations.

In this parameterisation, group 1 is considered to be the baseline group with mean µ1 and the mean for group 2 is δ2 higher. There is a single indicator variable that 'turns on' the parameter δ2 for group 2, but not for group 1.

The parameter δ2 is the difference between the mean response in group 2 and that in group 1 (the baseline group).


Matrix representation of the model

The next diagram shows this general linear model in matrix form.

Click on any y-value to see how the indicator variable adds the term δ2 only to observations in group 2.