Question: Kruuk et al. (1989) used a stratified sample to estimate the number of otter (Lutra lutra) dens along the 1400-km coastline of Shetland, UK. The coastline was divided into 242 (237 that were not predominantly buildings) 5-km sections, and each section was assigned to the stratum whose terrain type predominated. Sections were then chosen randomly from the sections in each stratum. In each section chosen, investigators counted the total number of dens in a I 10-m-wide strip along the coast. The data are in the file otters.dat. The population sizes for the strata are as follows:

          Stratum                    Total Sections             Sections Counted

1     Cliffs over 10 m                    89                                 19

2     Agriculture                          61                                  20

3     Not 1 or 2, peat                   40                                  22

4     Not 1 or 2, nonpeat              47                                  21

a Estimate the total number of otter dens along the coast in Shetland, along with a standard error for your estimate.

b Discuss possible sources of bias in this study. Do you think it is possible to avoid all selection and measurement bias?