[R] forcing a zero level in contr.sum

Peter Dalgaard
2010-07-08T08:08:51Z

Bond, Stephen wrote:
> I need to use contr.sum and observe that some levels are not
> statistically different from the overall mean of zero. What is the
> proper way of forcing the zero estimate? It seems the column
> corresponding to that level should become a column of zeros. Is there
> a way to achieve that without me constructing the design matrix?

As Berwin (indirectly) points out, you probably overlooked the how.many
argument to C().

However, are you _sure_ that you want to do this? This is like testing
that one treatment is exactly equal to the average of all other
treatments, which is a rather strange hypothesis.

--
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com