There are four main areas of this section and you can access them at any time using the navigation bar on the left. The four sections are:

• Explanation - a guide to data collection and sampling methods
• Illustration - illustrations of degrees of freedom and sample means
• Worksheets - a range of worksheets on basic sampling techniques
• Review - a brief summary of the topics covered in the 'sourcing' section and a final activity

When you are analysing an entire population, that is, all the members of the group you're studying, you are equally interested in data on every member of the group.

But sometimes the population you are interested in will be far too large for you to measure each of the members, (for example all the small and medium sized businesses in a region, all UK voters, all cars manufactured in Sunderland, or all freight containers entering the port of Dover). When this is the case, it is best to focus your analysis on a relatively small selection taken from the population. This selection is called a sample.

Using a sample helps you study a large population and learn things about it, so you can draw important conclusions about a big population without having go to the trouble of collecting data from every member. But to do this you have to understand something about the relationship between the information you have collected from the sample and the total population itself from which the sample is taken.

In this way, then, if you have measured data on the voting intentions of 2000 people, you will know if you can make conclusions about the voting intentions of the population in general.

As you go through this section and do the activities and tasks here, you will also know the limits that will exist on the conclusions you can safely make; the things you are not entitled to conclude about the wider population; tests that you can carry out on claims made about entire populations on the basis of a single sample; and how the validity of your conclusions may change as the size of your sample increases.