Appendix B.

Random sampling using a grid system.

When using random sampling techniques, large numbers of samples/records are taken from
different random positions within a habitat. A quadrat (a square frame) is most often used
for random sampling. It is used to obtain samples from areas of consistent size. The
actual size of the quadrat is determined by the habitat being sampled and by the purpose
of the survey.

In the simplest form of random sampling, the quadrat is thrown to fall at
‘random’ within the site. However, this is unsatisfactory because a personal
element enters into the throwing and it is never completely random. True randomness is an
important factor, because many of the common statistical techniques used to process
results are only valid on data that is truly randomly collected. This technique would also
only be possible where quadrats of small size are being used. It would not be possible to
throw anything larger than a 1m quadrat and even this might pose difficulties.

A better method of random sampling is to map the area and then to lay a numbered grid
over the map. A (computer generated) random number table is then used to select which
grids to sample in.

In this survey, a numbered grid was drawn on paper to represent the 20m square to be
randomly sampled. Each square on the grid represents a 2m square quadrat in the sampling
square. Ten consecutive random numbers (less than 100) were chosen from a random number
table. The quadrats in the grid bearing these numbers were then sampled.