8. Sampling

Sampling

• • Method of collecting data about a large group by taking a sample

  • Sample: A small group within the large group (i.e. total population)

  • Used to make conclusions about the total population

  • Before sampling, need to decide on the interval (based on sample size) used to select members of the sample

  • Intervals can be in terms of:

    • Distance (e.g. every 2 metres along a coast)

    • Time (e.g. every 30 minutes at a traffic junction)

    • People (e.g. every fifth person who passes a particular place in the neighbourhood)

  • Two common types of sampling are systematic sampling and random sampling

Sampling (Systematic sampling)

• • Collecting data at regular intervals

  • Involves pre-selecting a number for the interval (e.g. 2 metres)

  • Useful when investigating changes e.g. along a river

  • Sampling (Systematic sampling)

  • Easy to carry out

  • But may miss out variations or changes in the total population  biased data

  • E.g. By choosing equally spaced samples to study changes in a waterway, may miss the effect of canals joining the waterway

Sampling (Random sampling)

• • Collecting data at random or irregular intervals

  • The steps for random sampling are:

    • Decide how many samples needed

    • Obtain random numbers, either from a printed random number table or from an online random number generator

    • Use the random numbers to locate the samples

  • E.g.

    • Three water samples needed from a river

    • Random numbers are 20 and 17

    • Water sample would be collected at the 2-metre mark along the river, then at 1 metre after that (2 + 1), and 7 metres after that (2 + 1+ 7)

    • Water sample would be collected at 2 metres, 3 metres and 10 metres from the starting point

  • Best used when the population is the same throughout

  • Fair way of choosing (every member has the same chance of being picked)

Important to get a truly representative sample (same characteristics as the population) so that data obtained is accurate

Advantages

Saves time and energy

Essential because it is sometimes not possible to collect data about the total population

E.g. some people may decline to be surveyed

Disadvantages

May result in the data collection sites being widely spaced or concentrated in one area, leading to biased data

May not accurately represent the whole population