Groups of 20 snails were held for periods of 1, 2, 3 or 4 weeks (exposure)
in carefully controlled conditions of temperature (3 levels) and relative
humidity (4 levels). There were two species of snail, A and B, and the
experiment was designed as a 
completely randomized
design. At the end of the exposure time the snails were tested to see if
they had survived; this process itself is fatal for the animals. The
object of the exercise was to model the probability of survival in terms of
the stimulus variables, and in particular to test for differences between
species.
The data is unusual in that in most cases fatalities during the experiment were fairly small.