Students wanted to estimate the average vegetation cover on an area of heathland. They divided a 100 m × 100 m study area into a 10 × 10 grid and used random sampling to choose where to place 10 quadrats. Figure 1 shows the sample points they used.
Evaluate this sampling strategy for their enquiry.
Model answer
Random sampling has a clear strength: the students did not choose where to put the quadrats, so there is no bias from, for example, picking places that look interesting or are easy to reach. In principle every part of the heath had an equal chance of being sampled.
However, Figure 1 shows the weakness of random sampling with a small sample. All 10 points are in the western half of the area; no quadrat is east of 5 on the grid, so half the heath was not sampled at all. Several points are also clustered together, around 4 across and 2-3 up. If the vegetation in the eastern half is different, for example if there is a path, bracken or wetter ground there, the estimate of average cover will be wrong. Ten quadrats is also a small sample for an area of 10 000 m².
A better strategy would be systematic sampling, such as one quadrat in the middle of every other grid square, which would give even coverage, or simply more random points. If the heath has clear zones, stratified random sampling would make sure each zone is included.
Overall, random sampling is a reasonable choice in principle, but this sample is unrepresentative, so the estimate of vegetation cover is not reliable.