A Level

# Sampling

## 2. Sampling

Sampling is the process of collecting data from some sites or people in order to obtain a perspective on the population. It is applicable to both qualitative and quantitative methods. You should explain how representative a sample is. It affects how the findings can be applied.

### Sample

A limited number of things, such as a group of 100 people or 50 pebbles on a beach.

### Population

The total number of things, such as all residents of a city or all pebbles on a beach.

### Representative

How closely the relevant characteristics of the sample match the characteristics of the population.

### Bias

An inclination or prejudice towards or against a specific finding or outcome.

## Probability sampling

The aim of probability sampling is to select a sample which is representative of the population. There are three techniques:

### Random sampling

This is where each member of the population is equally likely to be included.

For taking random samples of an area, use a random number table to select numbers. Use pairs of numbers as x and y co-ordinates. You could use a metre rule interval markings (e.g. to take pebble samples on a beach) or grid references (e.g. to find random samples in a city).

### Stratified sampling

This is where a proportionate number of observations is taken from each part of the population.

For example, an urban ward may contain 8 deprived super output areas and 2 undeprived super output areas. A random sample may by
chance miss all the undeprived areas. By contrast, with a stratified sample, you can make sure that 80% of your samples are taken in the deprived areas and 20% in the undeprived areas.

### Systematic sampling

This is where observations are taken at regular intervals.

For example, every 10 metres along a line running from seashore inland across a beach, or recording the age of every fifth person in a shopping centre.

## Sampling over space

Point sampling. Take observations at individual points, like separate houses on a street

Line sampling. Take observations along a line, like up a beach from sea shore to base of the cliffs

Quadrat sampling. Make a square on the ground or on a map. Take observations within that square. For example, place a quadrat on the beach and measure pebble shape and size within that area.