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Often the relationship between response and predictor has been recorded for several levels of a grouping variable. Examples are:

• enzymatic reaction rates for a range of substrate concentrations recorded for each of two treatments
• dry matter weight of plants measured for a number of herbicide doses recorded for each of two herbicide treatments

We refer to this type of data as grouped data. One reason for considering grouped data is that data pertaining to one experiment should be analysed in one model, as this allows comparisons among groups at various levels.