What is a representative sample and why would I want one?
A representative sample is a sample that reflects the demographic distribution of a given (often national) population, with the aim of making your research findings more generalisable.
- If you were a psychologist investigating loneliness (or any other construct), you might use a representative sample in order to better estimate how loneliness affects the entire country.
- If you were a market researcher collecting feedback on product concepts, you might use a representative sample to ensure your survey reaches participants from a range of backgrounds, giving you a better idea of how the public as a whole might receive your ideas.
- If you were a political scientist interested in the outcome of an upcoming poll, you might use a representative sample to collect opinions from a group resembling the country’s demographic makeup.
Ultimately, drawing conclusions from an unrepresentative sample limits the extent to which you can say "my findings are likely to hold in populations other than the one I’ve tested”.
How does Prolific’s representative samples tool work?
We currently offer representative samples of two national populations: the United Kingdom and the United States.
When you use a representative sample, we take the intended sample size and stratify it across three demographics: age, sex and ethnicity. We use census data from the US Census Bureau or the UK Office of National Statistics to divide the sample into subgroups with the same proportions as the national population. This means, for example, that a representative sample contains the same proportion of 28-37 year old Asian women as the national population (or at least, as close as we can deliver).
- Currently we estimate our maximum deliverable representative sample size to be around 2000 for the UK and 1500 for the US.
- The minimum representative sample size you can request is 300.
- You can apply a custom blocklist or the 'exclude participants from previous studies' screener to prevent certain participants from accessing your study. These are the only prescreeners that can be applied to a representative sample study.
- It is not possible to increase the number of places available on a representative sample study after it has been published.
How much do representative samples cost?
When you use a representative sample we charge an additional fee in addition to our standard service charge. This is a small fee per-participant that decreases exponentially as sample size grows. In other words, you get a bulk discount!
This fee is charged upon study publication (rather than when each submission is approved as with the regular service fee).
For an accurate quote, create a new study, set your sample size, and choose 'representative sample' at the audience step of study creation. There is further guidance on this here: Setting up a study on Prolific.
What are the limitations of a representative sample?
Using a representative sample means the distribution of age, sex and ethnicity in the final sample will be similar to the selected national population. But keep in mind that no sample can ever be 100% representative, so using a representative sample does not guarantee that your results will be perfectly generalisable to the population (although it does help to improve generalisability).
- The size of a sample determines the precision with which we can match the national population. For example, if a subgroup is 0.2% of the national population, and the sample size is 300, then technically this subgroup should be represented by 0.6 of a participant. Our stratification algorithm ensures every subgroup is represented by at least one participant, but naturally this means that smaller representative sample sizes do not match the intended national population perfectly. Furthermore, consider that a smaller sample size may result in a single participant representing an entire subgroup.
- Though a representative sample matches the distribution of the national population on age, sex and ethnicity, it does not mean the sample is representative of the national population on all demographics. For example, the participants in the sample will all be members of Prolific, which is obviously not the case for every person in the United Kingdom or the United States. We hope to add further demographics to our representative samples feature in the future, such as socio-economic status or highest level of education.
Want to know more?
Our Representative samples FAQ article explains more about how this feature works. We highly recommend reading this article too before launching your representative sample study!