Who the study is really about

1. The target population is the whole group your research is about, such as every Year 11 pupil in your local authority.
2. You can almost never test all of them, so you study a sample, the smaller group who actually take part.
3. A sampling method is the procedure you use to pull that sample out of the target population.

1. A representative sample holds the same mix of ages, genders, backgrounds and abilities as the population, in roughly the same proportions.
2. Only a representative sample lets you generalise, which means treating what you found in the sample as true of the whole population.

Random sampling

1. Number everyone on a complete list of the target population, then draw numbers from a hat or a random number generator until the sample is full, so that every person had an equal chance of being picked.
2. One strength is that chance alone decides, so the researcher cannot favour one type of person even accidentally.
3. A second strength is that the selection rule is fixed and can be written down, so anyone repeating the study can follow it.
4. One weakness is that you need a full and accurate list of the population, which for a group such as all UK teenagers you cannot obtain.
5. A second weakness is that people chosen at random can still say no, and the group left over is then no longer a random slice of the population.

Stratified sampling

1. Split the population into the subgroups that matter, such as year groups, work out what share of the population each one is, then select at random inside each subgroup until it fills that share of the sample.
2. One strength is that every subgroup appears in proportion to its size, so this is normally the most representative method.
3. A second strength is that the proportions are set deliberately, so no important group can be squeezed out by an unlucky draw.
4. One weakness is that you have to know the make-up of the population before you start, and that information is often unavailable or out of date.
5. A second weakness is that it is the slowest method to organise, because you run a separate random sample inside every subgroup.

Volunteer sampling

1. Advertise the study, for example with a poster in the common room, and use the people who put themselves forward, which is why it is also called self-selected sampling.
2. One strength is that it is quick and cheap, because participants come to you instead of you having to find anyone.
3. A second strength is that volunteers have chosen to be there, so they are willing and less likely to drop out.
4. One weakness is that people who volunteer tend to be a particular type, often more confident or with more spare time, so the sample is biased before you collect a single score.
5. A second weakness is that you cannot control who replies, so whole parts of the population may not appear at all.

Opportunity sampling

1. Approach whoever is available and willing at the time and place you are collecting data, such as the pupils sitting in the canteen at lunch, until you have enough people.
2. One strength is that it is the fastest and cheapest method, because you use people who are already in front of you.
3. A second strength is that it is often the only realistic choice for a school study with one lesson and no register of the wider population.
4. One weakness is that you only reach whoever was in that place at that time, so the sample reflects your convenience rather than the population.
5. A second weakness is that the researcher tends to approach people who look friendly or unhurried, which biases the sample again.

Choosing between the four methods

1. Random and stratified sampling buy representativeness, and pay for it with time and with the need for a full list of the population.
2. Volunteer and opportunity sampling buy speed, and pay for it with a sample that may not stand in for the population.

Turning this into marks

1. Describe the procedure rather than naming it, so write that you numbered every pupil on the register and drew 30 of those numbers from a hat.
2. Give every strength and weakness its reason, such as stratified sampling being representative because every year group appears in proportion to its size.
3. The mark most often lost is application, so tie your choice to the population described in the question.