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May 24, 2015 1:43 PM
(1266 views)

My understanding is this:

- A factor is categorical variable
- A covariate is a continuous variable

Why do JMP define and use covariate differently?

What is the correct definition of a covariate?

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Solution

The term 'covariate' is used differently in different contexts. You seem to be asking about a covariate in the context of design of experiments. In this context, a covariate is a variable or factor that affects the response but is not controlled like the other factors. Instead, it co-varies with the change in the factor levels from one treatment to the next.

If the covariate levels are known for each run, then it can be included in the response model like any other factor. If you can determine the covariate levels prior to conducting the experiment, then record these values in a separate data table and then Add Factor > Covariate in the design window. The number of rows in this table must be at least as large as the number of runs in the design.

If you cannot determine the covariate levels before conducting the experiment, then Add Factor > Uncontrolled. This way you will have a placeholder in the data table for recording the level during the run and you will be able to include this factor in the model specification.

Learn it once, use it forever!

1 REPLY

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The term 'covariate' is used differently in different contexts. You seem to be asking about a covariate in the context of design of experiments. In this context, a covariate is a variable or factor that affects the response but is not controlled like the other factors. Instead, it co-varies with the change in the factor levels from one treatment to the next.

If the covariate levels are known for each run, then it can be included in the response model like any other factor. If you can determine the covariate levels prior to conducting the experiment, then record these values in a separate data table and then Add Factor > Covariate in the design window. The number of rows in this table must be at least as large as the number of runs in the design.

If you cannot determine the covariate levels before conducting the experiment, then Add Factor > Uncontrolled. This way you will have a placeholder in the data table for recording the level during the run and you will be able to include this factor in the model specification.

Learn it once, use it forever!