[R] Contrast problem

ripley@stats.ox.ac.uk ripley at stats.ox.ac.uk
Wed Nov 27 22:05:58 CET 2002

On Wed, 27 Nov 2002, Jane McFerren wrote:

> I have a question regarding contrasts in linear models. I have a factor variable
> age with 4 levels, in the output of the linear model I want a coefficient for each
> level of age.

You can't get that, only 3 contrasts can exist for a 4-level factor.
By default you get second - first, third - first and fourth - first.

> I have made sure that age is a factor but am unsure how to put age into the
> model to ensure the model knows it is a factor an dto display all levels.
> I have tried doing
> fit<-lm(y~C(age)+sex.....) (the dot bits are the rest of the function which I have
> cut out as its not important here)
> but it only displays age as though it has one level

Is age really a factor (try is.factor)?

> what am I doing wrong and how can I get R to recognise its a factor?

as.factor(age) will ensure that.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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