A population is a group that we want to find information about. It might be a group of people or it could be simply a group of numbers.
A census is when information about every member of the population is collected. The disadvantage of this method is that if the population is large, it can be difficult to collect and process so much information.
A sample survey is when information is collected from a small representation of the population. For example, if we were trying to find out what Britain"s favourite TV show is, it would be impractical to ask everybody in the country (as a census would do). Instead, 1000 people might be chosen and asked.
A sampling unit is a person/object to be sampled. The sampling units must be defined in such a way that any one member of a population is not sampled more than once. For example, if we were trying to find out how many cars the average family owns, the sampling unit would be a household rather than an individual, because we wouldn"t want to ask two people from the same household.
A sampling frame is the collection of all of the sampling units. Ideally, this should cover the whole population.
The idea of random sampling is that each member of the sample frame has an equal chance of being selected.
One way of doing this is to assign each member of the sample frame a number. Random numbers are then generated (using a computer or from a table) and those members of the sample frame whose numbers come out are sampled.
Instead of choosing the members to be sampled using random numbers (which might be difficult and time consuming for large populations), systematic sampling uses a simple rule to choose people. For example, every 10th member of the sample frame could be selected.
Stratified sampling can be used when the population in question is split up into groups who are likely to behave differently. For example, if we were trying to find the nation"s favourite television programme, most children would probably like different programmes to most adults.
Each group is sampled separately and the results are put together.
In the television example, if children make up 20% of the population, we would make sure that children make up 20% of the total sample.
Quota sampling involves splitting the population into groups and sampling a given number of people from each group.
This method is easy to implement when carrying out market research. For example, if someone is interviewing people at a shopping centre, they may have been told to interview 50 men and 50 women. It doesn"t matter how they choose the 50, as long as they interview that many.
If there is no sampling frame (list of sampling units), the above sampling methods can"t really be implemented. Quota sampling might be the only real possibility.