1. Unit 3- Investigative Biology Topic 2- Experimentation
e) Sampling

2. Sampling- what you need to know
Where it is impractical to measure every individual, a representative sample of the population is selected.
The extent of the natural variation within a population determines the appropriate sample size. More variable populations require a larger sample size.
A representative sample should share the same mean and the same degree of variation about the mean as the population as a whole.
In random sampling, members of the population have an equal chance of being selected.
In systematic sampling, members of a population are selected at regular intervals.
In stratified sampling, the population is divided into categories that are then sampled proportionally.

4. Sampling
When collecting data about a population, it is obviously ideal to measure and make a record of every individual, but unless the whole population is very small this is not practical. To overcome this, researchers measure a representative sample of the population. This is called sampling.

5. All populations show natural variation
More variable populations will require a larger sample size
Less variable populations can be measured effectively with a small sample

6. What is a representative sample?
It should share the same mean as the population as a whole
It should share the same variation about the mean as the population as a whole

7. Random sampling X X X Ecological sampling
Self-selection of sampling sites is rarely truly random.
Researcher bias can occur even when trying to be random.
One way to overcome this is by mapping the area to be sampled and using a numbered grid to map out the area.
In random sampling, all members of the population have an equal chance of being selected.
Random sampling is usually used when the area being tested is very large or uniform, or if there is a limited timeframe in which to obtain results.
Using random selection of sample sites is important as statistical analysis is only effective when sampling is truly random.
X X X
Using a random number generator overcomes the risk of non-random sampling . The random numbers that are generated are applied to the mapped area being measured.

9. Members of a population are selected at regular intervals.
Systematic sampling
Members of a population are selected at regular intervals.
Line transect
A line transect can be made by placing a piece of rope marked at 0.5 m/1 m intervals or a long measuring tape along the area you wish to sample.
Species that are touching the line can be recorded continuously along the line or at regular intervals.

10. Systematic sampling Belt transect
A belt transect is similar to a line transect but can give a measurement of species abundance as well as the presence/absence of species. Quadrats are placed on either side of the line and species recorded at regular intervals along the line.

11. Stratified sampling
The population is divided into categories that are then sampled proportionally.
When sampling habitats, often they are not uniform. Within one sample area there may be areas of grass, bushes, small trees etc. In a marine environment, a transect through the tidal zones on a beach may have areas of bare rock as well as algae.
Random sampling may result in a disproportionate emphasis being placed on one type of ground cover if the sampling is always selected on grass or bare rock.
To overcome this, stratified sampling is used. The total area covered by each of the categories of ground cover is worked out and a proportionate number of samples is made in each area.

12. Stratified sampling
Example of heath land, which is not uniform. Stratified sampling would allow the correct proportion of the areas that are grass, bush, small trees or large trees to be sampled.
There is a standard formula for calculating the number of samples that should be allocated to each unit (eg grass, bush):
number of quadrats sampled in the area =
the area of the unit × the total number of quadrats to be sampled
total area of the habitat

14. Sampling- what you need to know
Where it is impractical to measure every individual, a representative sample of the population is selected.
The extent of the natural variation within a population determines the appropriate sample size. More variable populations require a larger sample size.
A representative sample should share the same mean and the same degree of variation about the mean as the population as a whole.
In random sampling, members of the population have an equal chance of being selected.
In systematic sampling, members of a population are selected at regular intervals.
In stratified sampling, the population is divided into categories that are then sampled proportionally.