An Ecological Sampling Exercise 
Processing & Interpreting Raw Data


Methods used to Gather the Raw Data

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Students from Axe Valley Community College carried out a stratified random sampling exercise on strips 1 - 4 on the Heathland Restoration Project site at Offwell in March. The purpose was to investigate whether there was any difference in the % cover of Bluebells in spring-cut strips, as opposed to autumn-cut strips. The exercise also tested whether there was any difference between the % cover of Bluebells in the top of the strips (which are more influenced by neighbouring woodland) and the bottom of the strips, which are more open.

The survey was carried out early in the spring, before the spring-cut strips were cut. At the time, the Bluebells were developing and were not yet flowering. This meant that care had to be taken not to misidentify sedges and woodrushes (which are also common in all the strips) as Bluebells. These can look quite similar to Bluebells in the vegetative stages.

wpeBB.jpg (47211 bytes) Groups of students were assigned to sample the different strips for Bluebells. They surveyed ten random 1m2 quadrats in section 1. Ten random quadrats were also sampled in the top (western) half of each of sections 2 - 4, as well as in the bottom half of each of these sections. (Strip 1 is much smaller than the other strips and so was not divided into top and bottom halves. Diagram) A total of 70 quadrats were therefore sampled.

Students sampling the bottom of strip 4 (left)


The random quadrat locations were chosen using a random number table to select x and y co-ordinates to walk. A line across the centre of each of the strips acted as the x axis.

For example, if the first random number selected was 15, then a student would walk 15 paces along the centre line, beginning at the boundary of the relevant strip. If the next random number was 6, the student would then turn 900 and walk six paces up. At this point the quadrat would be laid down and sampled. When students were sampling the bottom half of the strips they would walk the required number of paces down 900 from the centre line, rather than up.

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Sampling involved determining the percentage cover of Bluebells in each quadrat. In each group, the same person estimated the % cover each time to reduce the effect of observer bias.

Now try your skills at processing and analysing the raw data they collected, by carrying out the exercise devised for the Axe Valley students by Dr Wendy Graham.

Continue to the follow-up Exercise




Thanks are due to Dr Wendy Graham and the Axe Valley Community College,
for permission to publish the raw data and follow-up exercise.