7.5 Required practical: sampling with quadrats and transects
6 exam questions worth 14 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Explain why quadrats should be placed randomly when estimating a population.
Mark scheme
- to avoid bias / so results are representative (1)
- every part of the area has an equal chance of being sampled (1)
Model answer
Placing them randomly avoids bias, so the sample represents the whole area.
Describe how to place quadrats randomly in a field.
Mark scheme
- lay out tape measures to make a grid / axes (1)
- use random numbers (from a generator / table) as coordinates (1)
Model answer
Lay two tape measures at right angles to make a grid, and use a random number generator to choose coordinates for each quadrat.
A student counted dandelions in 10 quadrats, each 0.25 m2. The mean was 4 dandelions per quadrat. The field has an area of 500 m2.
Estimate the number of dandelions in the field.
Mark scheme
- 500 ÷ 0.25 = 2000 (quadrats) (1)
- 4 × 2000 (1)
- = 8000 (1)
Model answer
Number of quadrats that fit in the field = 500 ÷ 0.25 = 2000. Population = 4 × 2000 = 8000 dandelions.
Describe how a student could use a transect to find out how the number of daisies changes from the shade of a tree into an open field.
Mark scheme
- lay a tape measure from the tree into the field (1)
- place quadrats at regular intervals along it (1)
- count daisies and measure light intensity at each point (1)
Model answer
Lay a tape measure from the base of the tree out into the field. Place a quadrat every 2 m along it, count the daisies in each, and measure the light intensity at each point with a light meter.
Capture-recapture: 50 beetles were caught, marked and released. Later, 40 were caught, of which 10 were marked.
Estimate the population of beetles.
Mark scheme
- (50 × 40) ÷ 10 (1)
- = 200 (1)
Model answer
population = (50 × 40) ÷ 10 = 200 beetles.
The counts in 7 quadrats were 2, 5, 3, 5, 6, 1, 5. Find the median and the mode.
Mark scheme
- median 5 (1)
- mode 5 (1)
Model answer
In order: 1, 2, 3, 5, 5, 5, 6. The median is 5 and the mode is 5.
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