R help - Feb 2010 - What are Type II or III contrast? (contrast() in contrast package)

?contrast in the contrast package gives me the following description.
However, I have no idea what Type II and III contrasts are. Could
somebody explain it to me? And what does 'type' mean here?

    *?type?*: set ?type="average"? to average the individual contrasts
    (e.g., to obtain a Type II or III contrast)

On Feb 2, 2010, at 11:38 PM, Peng Yu wrote:
> ?contrast in the contrast package gives me the following description.
> However, I have no idea what Type II and III contrasts are. Could
> somebody explain it to me? And what does 'type' mean here?
>
>    *?type?*: set ?type="average"? to average the individual
contrasts
>    (e.g., to obtain a Type II or III contrast)
In no particular order:
http://courses.washington.edu/b570/handouts/type3ss.pdf
http://core.ecu.edu/psyc/wuenschk/SAS/SS1234.doc
http://n4.nabble.com/a-kinder-view-of-Type-III-SS-td847282.html

Don't expect any follow-up questions to be answered or further  
citations offered. This is really more in the realm of statistics  
education than an R specific question.
>
-- 

David Winsemius, MD
Heritage Laboratories
West Hartford, CT