[R] Binomial glms with very small numbers
Spencer Graves
spencer.graves at pdf.com
Thu Jan 15 02:15:04 CET 2004
The advisability of using "glm" with mortality depends not on the
size of sample groups but on the assumption of independence: Whether
you have 3 individuals per group or 30 or 1, is it plausible to assume
that all individuals represented in your data.frame have independent
chances of survival give the potentially explanatory variables? If the
answer is "yes", then "glm" is appropriate. If the answer is "no", then
some other tool may be preferable. However, "glm" is quick and easy in
R, and I might start with that, even if I felt the assumption of
independence was violated. If I found nothing there, I would not likely
find anything with techniques that handled more appropriately the
violations of independence.
Similarly, I can't see how it would matter whether potentially
explanatory variables were continuous or categorical, as long as a
categorical variable were appropriately coded as a factor (or
"character", which is then treated as a factor) if it has more than 2
levels.
Hope this helps.
spencer graves
Patrick Connolly wrote:
>V&R describes binomial GLMs with mortality out of 20 budworms.
>
>Is it appropriate to use the same approach with mortality out of
>numbers as low as 3? I feel reticent to do so with data that is not
>very continuous. There are one continuous and one categorical
>independent variables.
>
>Would it be more appropriate to treat the response as an ordered
>factor with four levels? If so, what family would one use?
>
>TIA
>
>
>
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