2. Sampling methods
The aim of sampling is to select a sample which is representative of the population.
There are three techniques:
- Random sampling
- Stratified sampling
- Systematic sampling
This is where each member of the population is equally likely to be included.
For taking random samples of an area, use a random number table or random number generator to select numbers. Use pairs of numbers as x and y co-ordinates. You could use a meter ruler interval markings (e.g. to take vegetation samples in a saltmarsh) or grid references (e.g. to find random samples in an upland area).
Random sampling is used to select a sample that is unbiased. Within each area, every part of the area must have an equal chance of being chosen. Random sampling with quadrats is used to examine differences between contrasting habitats within an habitat. There are many possible questions that could be investigated in this way.
In each habitat you are investigating, mark out a 10m x 10m square on the ground by laying one tape measure 10m lengthways and a second tape measure 10m at right angles to the first. Use a random number table (or random number generator on a calculator) to select numbers from 1-10. Each pair of random numbers can be used as x and y co-ordinates, using the metre interval markings on each tape measure. Use these to locate the lower left hand corner of a frame quadrat or use a point quadrat. Take at least 10 quadrat samples in each area.
This is where a proportionate number of observations is taken from each part of the population.
Divide a habitat into zones which appear different and take samples from each zone. For example, if vegetation cover in an area of heathland is 60% heather and 40% gorse, for a stratified sample take 60% of the samples from within heather and 40% of the samples from within gorse.
Systematic sampling is used where the study area includes an environmental gradient. A transect is used to sample systematically along the environmental gradient. For example, every 10 meters along a line running from seashore inland across a sand dune system.