This article answers common questions about representative samples on Prolific, covering sample sizes, prescreener compatibility, participant recruitment methods, completion timeframes, options for slow-filling samples, and how to download your data once the study is complete.

Contents

We recommend you read the What are representative samples on Prolific article first, to understand what they are, what types are available on Prolific, where we source our data, and how we organize participant subgroups.

What are the minimum and maximum sample sizes?

The minimum sample size for any representative sample is 300 participants. The maximum sample size varies depending on the type of sample you select and is dynamic. We maintain a size limit to ensure we can fulfill the quotas for each demographic subgroup. This means you cannot run representative samples larger than the maximum size. Attempting to work around this by running multiple studies would likely reduce the overall representativeness of your data.

To check participant availability for each representative sample, fill out the study distribution section on your study setup page and select your representative sample criteria. The available number of participants will then appear in red in the 'Participants' section.

Can I apply screeners alongside a representative sample?

When using representative samples, you can only apply two types of prescreeners: a custom blocklist or the 'exclude participants from previous studies' screener. These options allow you to prevent specific participants from accessing your study.

No other prescreeners, including participant groups, can be used with representative samples. This restriction exists because the representative sample itself serves as a screening criterion - adding any additional screeners would compromise the representativeness of your sample.

You can, however, use custom screening, but it's important to note that doing so makes the sample no longer representative, so we strongly advise against it.

Who is eligible to be in a representative sample of the UK / US?

To be eligible for a representative sample, participants must have an available spot in their matching age, sex, or ethnicity subgroup. They must also be current residents of the country being sampled and fluent in that country's language.

How are participants recruited to my representative sample?

Participants join representative sample studies exactly as they would join regular Prolific studies. Representative samples are filled primarily on a first-come, first-served basis, though we have systems in place to ensure fair distribution of studies across our participant pool.

What type of allocation algorithm does Prolific use?

Standard representative sample

  • UK & US: Cross-stratifying on age (5 brackets), ethnicity (5 groups) and sex (2 groups) results in 50 subgroups: one for every combination of answers.

Political representative samples

  • US: Cross-stratifying on age (5 brackets), sex (2 groups) and political affiliation (3 groups) results in 30 subgroups: one for every combination of answers.
  • UK: Cross-stratifying on age (5 brackets), sex (2 groups) and political affiliation (7 groups) results in 70 subgroups: one for every combination of answers.

Regional representative sample

  • UK: Cross-stratifying on age (5 brackets), sex (2 groups) and region (12 groups) results in 120 subgroups: one for every combination of answers.
  • US: Cross-stratifying on age (5 brackets), sex (2 groups) and region (4 groups) results in 40 subgroups: one for every combination of answers.

All representative samples

Using census data, we can calculate the proportion of each subgroup in the national population. In order to ensure minimal representation, we first allocate 1 space per subgroup. We then allocate the remaining spaces in the sample proportionally, according to the national population.

Where we need to round subgroup sizes (i.e., proportionate allocation indicates a subgroup should have 2.6 participants), we try to round to the closest whole number, while balancing round-ups and round-downs to ensure the total sample size remains exactly what was asked for.

How long does it take to collect a representative sample?

Representative samples tend to take longer to recruit than Standard samples. Representative samples include demographics with smaller population numbers, so it can be hard to guarantee all demographics will be online daily to take your study. We're always striving to recruit more participants in niche demographics to improve this service for you. Typically, we expect a representative sample study to be completed within 2 to 4 days.

Note that applying a blocklist could slow down recruitment time.

What can I do if my sample is slow to fill?

If your representative sample study is still awaiting submissions 48 hours after launch, you'll have the option to reallocate your sample. This process loosens eligibility requirements for remaining places to balance timely data collection with sample representativeness.

Once you choose to reallocate your sample, you cannot undo this action or increase study places later.

Most representative samples reach at least 90% completion within 48 hours and fill completely after loosening requirements. This typically results in a sample accuracy of 95% or higher.

How does reallocating work?

Reallocating removes age bracket restrictions from all unfilled places in the sample.

For example, if you have 2 unfilled places for 28-37-year-old Asian women in your study 48 hours after publication, these places will be opened up to all Asian women in the country, regardless of age.

Additionally, the algorithm monitors the overall demographic distribution based on sex and ethnicity. This means that sometimes reallocated places may be reassigned to a different sex/ethnicity to maintain the most accurate representation of the population.

How can I calculate sample accuracy?

To calculate the final accuracy of your representative sample, sum the total number of subgroup requirements that participants in your final sample met, then divide by the total number of requirements that could have been met.

Example: You have a final sample of 1000 participants

  1. The total possible number of requirements your sample could meet is 3000. 3 (age, ethnicity, sex) x 1000 (the total sample size)
  2. 900 places have been filled by participants that met their subgroup’s requirements for age, ethnicity and sex: 900 x 3 = 2700 requirements met.
  3. 80 participants were collected after one round of reallocation: These participants met the requirements for ethnicity and sex: 80 x 2 = 160 requirements met.
  4. 20 participants were collected after two rounds of reallocation: These participants met the requirement for sex only: 20 x 1 = 20 requirements met.
  5. 2700 + 160 + 20 = 2880
  6. 2880 / 3000 = 96% sample accuracy.

How can I download the data after the study is complete?

You can export the Prolific demographic data at any time. You can find more information about exporting data in this article.

When you select a representative sample for study distribution, the relevant additional prescreeners ('Ethnicity (simplified)', 'Political affiliation', or 'Current UK area of residence'/'Current USA area of residence') are automatically added and included in your data export.

Can I run a representative sample for a country other than the UK or the US?

Currently, we only offer representative samples for the UK and the US. Running a sample for other countries is not possible at this time. If you’d like this option in additional countries, please let us know. Your feedback helps us understand demand and prioritize future developments.

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