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Statistical sampling methods
1,609
A CENSUS obtains information from
every member of a population.
A SAMPLE is a part of a population.
A sample is biased if it is not
representative of the population
A sample is random if it is chosen without conscious decision. This ensures each member of the population has an equal chance of being picked
Keywords
Simple Random Sample
Every item in the population has an equal chance of being chosen. eg use a calculator or pick names from a hat.
This method is suited to a relatively small population where a complete list of the population to choose the sample from exists.
Representative
Random
Stratified
Quota
Systematic
Convenience
Systematic Sample
Convenience Sample
Census
Population
Sample
Survey
Questionnaire
Biased
For example, a mixed group could be listed in alphabetical order and every sixth student selected, starting with the 3rd student.
This is a simpler and quicker method to select a sample, but may be unrepresentative if a pattern exists in the list.
The most convenient sample is chosen which, for a sample of size sixty, could mean the first sixty people you meet. It is highly likely that this sample would be biased .
Telephone and postal surveys are also highly unrepresentative.
When information is required about a large group of people it may not be possible to survey everyone, so we pick a sample.
The sample should be large enough to make the results meaningful and representative of the whole group.
Quota Sample
Stratified Sample
The population is divided into groups (gender, age, etc). A given number (quota) is surveyed from each group. This type of sample is not random, but is cheap to carry out and can be done quickly.
eg in market research they may question
50 men and 50 women
The science of surveys
Sampling
Let's discuss the pros and cons of each approach Mrs Megan considered taking
Let's practise working out
the numbers for some
stratified samples
In pairs, discuss these four situations
John was carrying out a survey to find how far, on average, residents in his town travel to work. He asked all the people at his local railway station one Monday morning.
Glowalot, a light bulb manufacturer claimed that their light bulbs lasted for more than 200 hours. Gina thought it would be a good idea to test their claim by lighting all the bulbs produced in one month.
Hazel thinks that boys at her school get more pocket money than girls. There are 300 children at the school, 120 boys and 180 girls. In her survey she asks 30 boys and 30 girls.
Catriona believes that more people in Scotland get married in church than in a registry office. She asks all the people attending a church service where they got married.