How to subset() from data frame using specific rows

But you do not loose them, your data frame is cut according to sites
variable and put into a list

I know this, Petr. But adding them to the database table ensures that the
information is there, too.

   This brings up another question, but I should put that on a different
thread. It's about process and work flow; that is, when can I use multiple
factors in the original data frame and when I need to split and subset the
data frame. I think it depends on how many factors can be specified by a
particular model or graph. Regardless, I'll hold off on this as I work
through these initial exploratory steps.

Thanks,

Rich