How many participants do I need? (Sample size)

How many participants do you need for your study? This is your “sample size”. We recommend different sample sizes for each technology.

Eye Tracking

We recommend at least 25 successful completes with high accuracy for ideal Eye Tracking results. Eye-tracking is more objective and easily measured; most variations in behaviour will be captured with 25-35 samples for most content (e.g. a 30 second ad).

Emotion Facial Coding

We recommend at least 50 and ideally 75 completes for robust facial coding. A sample of 100 is preferred. Larger samples improves detection of weak effects and reduces noise or anomalous results.

Implicit association Testing

We recommend a sample of at least 50 and ideally 100 participants for statistical robustness.

Summary

Since Eye-Tracking has a 50% success rate, you can meet these requirements for most projects by attracting a sample of approximately 100 participants who pass screening. A typical approach for a single Stim would be to screen 200 participants, accept 100, perform implicit testing prior to Stim viewing, eye-tracking and facial coding during Stim viewing, and a second implicit test on the sample after exposure to measure effects.

Note that if you wanted to test two Stims independently, you would need twice as many participants.

If you are not sure about your sample size, please contact us for advice.

Success Rate

The other consideration when recruiting participants is the success rate. The success rate depends on three main factors:

  • The motivation and familiarity of your viewers
  • Any control you have over the computer or location used
  • Your ability to screen-out viewers who do not meet all the criteria

These factors will be affected by the method you use to recruit viewers.

If you do not screen viewers, they are poorly motivated, or they are suspicious of your project or participant briefing, as many as 50% will simply decline to participate. Therefore, the recruitment strategy is very important. You may want to engage a reputable panel provider.