A biology student came to me with a data analysis situation that I wasn't sure how to
deal with. Sound advice would be appreciated.
Scenario:
Ben has a number of 1 meter square plots where he placed one or more seeds:
50 plots with 1 seed
10 plots with 25 seeds
10 plots with 50 seeds
He replicated that design with four species of seeds.
He visited the plots every day for a week to count the number of seeds remaining.
So the questions of interest are:
a) Does density have an effect on seed survival?
b) Does species have an effect on survival?
c) What does the data look like over time?
We considered modelling/analyzing the data in two ways (using SAS):
1) with seed as the unit of analysis, using Proc Lifetest to generate survival curves.
The problem: for medium and high density plots, seeds would not seem to be
independent.
2) with plot as the unit of analysis, using GLM to get a mixed model, where
time is a repeated measure and density and species are between groups factors.
Problem: low density (1 seed) plots have a dichotomous outcome, so much
of the data is non-normal.
Any suggestions? Thanks.
rick barton
