Inclusion and exclusion criteria

Identify a target population that applies to your research question.

Your target population should be the population that you wish to benefit through the findings of your research.

• For example, in a study that looks at the effectiveness of a new drug in lowering blood pressure, an appropriate target population may be ‘all patients with high blood pressure’.
• People with normal or low blood pressures are unlikely to benefit from your research and thus would not be an appropriate target population (unless you can justify the contrary).

Your sample of subjects should be generalisable to this target population.

Your proposed sample should be representative of your target population.

• For example, if your target population is ‘all patients with high blood pressure’, your sample’s age range, male-to-female ratio, ethnicities, co-morbidities, occupations, income range etc. should be similar to those of the general population of people with high blood pressure. ⁽¹²⁾
• You should not disproportionately include more males than females, or more people from one ethnicity over another.

Consider the vulnerabilities of your sample.

Some people are generally considered to be more vulnerable than others.

• For example, children, elderly, people with physical or mental disabilities, people who lack capacity.

While some vulnerabilities are context-specific.

• For example, if a possible side effect of the experimental blood pressure drug is abnormal heart rhythms, then people with existing heart rhythm problems may be more vulnerable to harm than others without.

And put in specific measures to minimise those vulnerabilities.

For example:

• If your sample is likely to consist of elderly people (as they are more likely to have high blood pressure), you may want to make your consent form easy to read with large fonts, and allocate more time when obtaining consent.
• If your study drug may cause abnormal heart rhythms, you may want to screen potential subjects for existing heart rhythm abnormalities before you include them, and monitor all subjects with an electrocardiogram (ECG) throughout the study.

You may exclude certain people if their risks are considered too high and not worth any potential direct therapeutic benefits that they might gain from the study.

• For example, if your study drug may cause abnormal heart rhythms, you may want to exclude people who have recently had a heart attack or stroke. This is because abnormal heart rhythms may increase their risks of suffering another heart attack or stroke.

However, exclusion from research may reduce the generalizability of your research findings to these groups of people. Whenever possible, you should try to put in sufficient protective measures to minimise their risks while enabling their participation.

Your sample size should allow your study to be adequately ‘powered’, but not bigger than necessary.

You should be able to justify your target sample size.

• For example, by performing statistical calculations.