1. Sampling Great Marlow School Mathematics Department

2. Domesday Book In 1085 William I wanted to know how much money he could raise in taxes. He needed to know about the people in England. William ordered a nationwide survey of his people. The results were written down in the Domesday Book. This was probably the first census. These days the National Census is carried out every 10 year by the government. Every household in the country completes a questionnaire. Great Marlow School Mathematics Department

3. Census Data Census data obtains information from every element of a population. A population is everybody (people) or everything (objects) in the study. Census information is used by central and local government, health authorities and many other organisations to allocate resources and plan services for everyone. A list of all items of the population is called the sampling frame. Great Marlow School Mathematics Department

4. Sample Data When a census is not practical you need to take a sample of the population. The purpose of sampling is to collect data from some of the population and use it to make conclusions about the whole population. To collect sample data, you take information from part of the population. Two samples drawn from the same population are likely to give different results. Great Marlow School Mathematics Department

5. Bias You should try to ensure that the sample you choose is free from bias. Bias in sampling arise from: Misidentifying the population. Choosing an unrepresentative sample. Non-response to a survey Asking ambiguous or leading questions. Dishonesty of those sampled. Errors in recording answers. Not controlling external factors in experiments, such as the effect of noise on the ability to complete a task requiring concentration. Great Marlow School Mathematics Department

6. Avoid Bias Anything that distorts data so that it does not fairly represent the population is called bias. You need to take care to avoid bias in the way that you collect data. Otherwise your results are not reliable. Great Marlow School Mathematics Department

7. Sampling Methods Random sample Systematic sample Convenience sampling Stratified sample Quota sampling Cluster sampling Opinion polls Great Marlow School Mathematics Department