1. Selecting Research Participants
Chapter 6
Selecting Research Participants

2. Selecting research participants
In research you select a sample from a population of potential participants.
Census – the whole population is researched
Inferential statistics – used to make statements (inferences) about the population based on the findings from our sample.
Sampling frame – a list of the population from which the sample is drawn.

3. Sampling methods Probability Sampling Non-probability Sampling
Techniques for which you can specify the probability that a participant will be selected from a population.
Non-probability Sampling
It is impossible to specify the probability of selecting any one individual.
The sample may or may not be representative of the population.

4. Probability sampling
Random sampling – a sample is drawn such that each member of the population has an equal probability of being included in the sample.
vs. random assignment – requires that participants have been independently assigned to groups.

5. Probability sampling
Systematic sampling – the population size is divided by your sample size to provide you with a number, k, for example; then from a random starting point you select every kth individual.
Stratified sampling – the population is divided into strata based on some population characteristic and participants are randomly selected from each stratum (therefore each stratum is proportionally represented in the sample).

6. Probability sampling
Cluster sampling – can be used when a population list is not available and researchers simply identify a number of clusters or groups and include all participants in the cluster/group in the sample.
Multistage sampling – a cluster technique where smaller clusters are randomly selected from larger clusters that were randomly selected previously.

7. Non-probability sampling
Convenience sampling – using whatever participants are easily available.
Quota sampling – convenience sampling in which the goal is to select participants with particular characteristics until you have enough.
Referral sampling – involves including participants in the sample who have been referred by other participants.

8. Sample and effect size Sample size depends on:
the power of the statistic
your research design (how many conditions you have)
size of the effect
variability of the data

9. Power revisited
You can increase power by increasing the number of participants in your sample.
When deciding which sampling method to use consider:
How much time do I have?
How much money do I have?
How much help can I get?

10. Describing Your Participants
How many?
How Selected?
Where do they come from?
What age?
max & min, mean and s.d
Composition
Gender breakdown
Ethnicity
Other important characteristic related to hypotheses
Inclusions/Exclusion
Compensation