Binomial glms with very small numbers
Yes, but "glm" maximizes the binomial likelihood assuming
log(p/(1-p)) is a linear model. Therefore, you don't have to transform
the 0's and 1's. There are cases where a particular combination of
potential explanatory variables will clearly separate mortalities from
survivors. I don't know that the algorithm does with such cases, but it
should send a slope essentially to infinite. However, if you don't have
this case, "glm" should do what you want.
hope this helps. spencer graves
Patrick Connolly wrote:
On Wed, 14-Jan-2004 at 05:15PM -0800, Spencer Graves wrote: |> 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 I think we can assume independence. What concerned me more was the fact that there will be rather a lot of 0s and 1s, corresponding to -Inf and Inf on the transformed scale. Only half the possible values (namely, 1 & 2) will be usable in the fitting. On second thoughts, since the response can be given as a binary, perhaps I was unnecessarily concerned. |> 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. Thanks for that suggestion. |> |> 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. I didn't think it would make a difference but I included it in case someone more knowledgeable had reasons why it did. Thanks.