7.6. To study the relative survival capacities of two species of native and exotic snails, here labelIed A and B, groups of 20 animals were held in controlled laboratory conditions for periods of 1, 2, 3 or 4 weeks. At the end of the period the animals were checked for whether they had survived, but as the check itself is a destructive process a longitudinal study with the same animals was not possible. The groups were held in chambers where the temperature and relative humidity were held fixed at three and four levels, respectively. There were thus 2 x 4 x 3 x 4 = 96 groups laid out in a complete factorial design.

The data are shown in Table 7.5, where each entry is the number who did not survive out of the 20 test animals. The data set is also available as the data frame snails in library MASS. Variable Species is a two-Ievel factor but treat the other stimulus variables as quantitative.