- Random sampling
- Every member of the target population has an equal, independent probability of being selected — achieved via random number generators applied to a complete sampling frame. Tends to produce representative samples and allows generalisation from sample to population. Main limitation: obtaining a complete sampling frame of a large real-world population is typically impossible, and small random samples can still deviate substantially from the population by chance.
- Opportunity (convenience) sampling
- Recruiting whoever is accessible at the time and place of the study. The most commonly used method in psychology because it is fast and inexpensive. The critical limitation is that the sample reflects whoever happened to be in that place at that time — often over-representing university students, leading to the WEIRD problem (Western, Educated, Industrialised, Rich, Democratic) and limiting generalisability.
- Stratified sampling
- Divides the population into strata (subgroups based on characteristics relevant to the research question: age, gender, ethnicity, socioeconomic status) and randomly samples proportional numbers from each stratum. Ensures subgroups are represented in proportion to their presence in the population, producing more representative samples than simple random sampling when strata are relevant to the variables studied.
- Volunteer (self-selected) sampling
- Participants come forward in response to an advertisement or call. Introduces volunteer bias: people who actively volunteer for research tend to differ from the general population — often more curious, more altruistic, or more motivated by personal experience with the topic. Ethical by design: participants have consented before the study begins.
- Informed consent
- Participants must receive sufficient information about the study's nature, procedures, risks, and their right to withdraw to make a genuinely voluntary decision about participation. Information must be comprehensible, not buried in jargon or given under social pressure. Additional safeguards (parental consent, assent) are required for vulnerable populations including children and individuals with cognitive impairments.
- Right to withdraw
- Participants may stop at any time during the study without requiring a reason, and without suffering any negative consequence. The right extends retrospectively: after the study ends, participants may request their data be excluded from analysis and destroyed, even when data collection is complete. This is particularly important in studies using deception, where participants only fully understand what they took part in during the debrief.
- Deception in research
- Some research cannot be conducted without deception because revealing the true purpose would fundamentally alter participants' behaviour. The BPS permits deception only when: the study has genuine scientific value that cannot be achieved otherwise, there is no reasonable alternative, likely harm is minimal, and a thorough debrief is provided. Deception about aspects of participation that participants would have objected to — such as the true risks involved — is not permissible.
- Debriefing
- A post-study conversation explaining the true purpose of the study, addressing any deception used, checking for distress, providing support if needed, and offering the option to withdraw data. Not a tick-box exercise: if a participant is distressed, the debrief must address this. The BPS requires that participants leave no worse off than before they took part.
- Protection from harm
- Researchers must take all reasonable steps to protect participants from physical and psychological harm during and after the study. The threshold is that participants should not experience greater risk than they would in everyday life. If a study produces unexpected distress, the researcher must be prepared to stop the study, provide appropriate support, and refer participants to professional help if necessary.