11.2 Types Of Sampling

Question 1

What is Sampling

Answer 1

Sampling means taking measurements of a limited number of individual organisms in a particular area.

Question 2

Random Sampling

Answer 2

Means selecting individuals by chance. In a random sample, each individual in the popn. has an equal chance of being chosen.

To decide which organisms to study, random number tables or computers can be used.

To take a random sample at a grass verge you could:

1) mark out a grid on the grass using 2 tape measures laid at right angles.
2) use random numbers to determine the x coordinate and the y coordinate on your grid.
3) take a sample at each of the coordinate pairs generated.

Question 3

Non-random Sampling

Answer 3

Alternative sampling method where the sample is not chosen at random. Can be divided into 3 main techniques:

1) opportunistic - weakest form as it may not be representative of popn. Uses organisms that are conveniently available.
2) stratified - some popn.s can be divided into a number of strata based on a particular characteristic. For example, male and female. A random sample is then taken from each strata group.

3) systematic - different areas within an overall habitat are identified which are then sampled separately.
Systematic sampling is often carried out using a line or belt transect.

Question 4

Line and Belt Transect

Answer 4

Line = involves marking a line along the ground between 2 poles and taking specified points, this can include describing all organisms which touch the line.

Belt = provides more info; 2 parallel lines are marked and samples are taken of the area between the 2 lines.

Question 5

Reliability

Answer 5

A sample is never entirely representative of the popn. Possibly due to:

1) sampling bias - selection process may be biased. May be by accident or deliberately. Effects of sampling bias can be reduced using random sampling where human involvement in choosing the samples is removed.

2) chance - organisms selected may, by chance, not be representative of whole popn. Chance can never be completely removed from the process but it’s effect can be minimised by using a large sample size.
The greater the number of individual studied, the lower the probability that chance will influence the result.
Therefore more reliable if you have a larger sample size.