SAMPLING If we need to carry out a survey, it is not always possible to test a whole 'population'. In cases such as this, a method of sampling is needed.

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Presentation transcript:

SAMPLING If we need to carry out a survey, it is not always possible to test a whole 'population'. In cases such as this, a method of sampling is needed.

RANDOM SAMPLING Use a table of random numbers. Use the random numbers function on a calculator or computer

STRATIFIED SAMPLING The sample size is proportional to the size of the 'layer of the population. You can choose a percentage of the population or a set number for the size of the sample.

Stratified Sampling – Example I want a sample of 200 pupils from a school. The table below shows the number of pupils in each year group. YearNo. of pupils How many year 7 pupil should be in my sample? There are 1035 pupils altogether 240 x 200 = > 46 pupils 1035 You then choose 46 pupils by random sampling

Stratified Sampling – Question I want a sample of 150 pupils from a school. The table below shows the number of pupils in each year group. YearNo. of pupils Work out how many pupils from each year group should be in the sample of 150 pupils.

SYSTEMATIC SAMPLING If a sample of size s is to be taken from a population of size n, then every n/s member of the population is tested. The start point is chosen at random. If we want to test a sample of size 100 from a population of 2000, we test every 2000/100 = every 20 th member. We use random numbers to determine the starting point. E.g. If we obtained the random number 7, then we would test 7, 27, 47, 67,... Make sure that each member of the population is arranged randomly. If they are grouped together before the sample is taken, then the sample could become biased.

QUOTA SAMPLING It is a method often used by market research companies. The interviewer will normally be given some instructions (e.g. ask approximately the same number of males and females between the ages of 20 and 60), but they will then be left to choose the interviewees themselves.

AVOIDING BIAS State which sampling method you have chosen to use and why you have used it. Evaluate your sampling process by highlighting problems which may arise from your selected sampling technique. A survey to find out what your local town's favourite pet is will reveal biased results if you choose to ask passers by in the park because you are most likely to find people walking their dogs.