Your screening criteria directly affect how easy (and expensive) it is to recruit participants. Understanding incidence helps you plan effectively.
What Is Incidence Rate?
Incidence rate = percentage of people who qualify for your study.
Example:
Screening for smartphone owners: ~85% qualify (high incidence)
Screening for specific brand owners: ~10% qualify (low incidence)
Screening for heavy users of niche product: ~2% qualify (very low incidence)
How Incidence Affects Recruitment
Incidence | Difficulty | Impact |
70%+ | Easy | Fast recruitment, lower cost |
50-70% | Moderate | Standard timeline |
30-50% | Challenging | Longer recruitment |
10-30% | Difficult | Higher cost per response |
Below 10% | Very difficult | Much higher cost, slow |
Calculating Combined Incidence
Criteria compound when combined:
Criterion | Individual Incidence |
Age 25-34 | 20% of adults |
Female | 50% of population |
Has children | 40% of adults |
Combined: 20% × 50% × 40% = 4%
Only 4% of the general population matches all three criteria.
Factors That Lower Incidence
Demographics
Add This | Impact |
Specific age range | ÷ 3-5 typically |
Single gender | ÷ 2 |
One country | Varies |
Regional targeting | ÷ 2-10 |
Screening Questions
Criterion | Typical Incidence |
Purchased category recently | 20-40% |
Uses specific brand | 5-30% |
Heavy user (daily+) | 10-30% |
Purchased specific product | 2-15% |
Rare behavior | Below 5% |
Managing Low Incidence
Option 1: Broaden Criteria
Relax non-essential requirements:
Widen age ranges
Include more locations
Use longer timeframes ("past 6 months" vs "past month")
Option 2: Accept Longer Timelines
Low incidence means:
More people screened per completion
Recruitment takes longer
Plan accordingly
Option 3: Increase Budget
Lower incidence typically means higher cost per completed response.
Incidence Scenarios
Consumer Products (High: 70-80%)
"Adults 18-65 who purchase groceries"
Easy to recruit, broad audience.
Category Users (Medium: 20-35%)
"Adults 25-54 who purchased coffee pods in past 3 months"
Moderate difficulty, good targeting.
Brand Specific (Low: 3-10%)
"Adults 25-44 who purchased Brand X in past month"
Challenging, very specific audience.
B2B Decision Makers (Very Low: 1-5%)
"IT managers at 100+ employee companies who make software decisions"
Difficult, highly targeted.
Best Practices
💡 Tip: Estimate before launching. Calculate expected incidence to set realistic timeline and budget expectations.
💡 Tip: Prioritize criteria. Know which are must-haves versus nice-to-haves. Remove nice-to-haves if incidence is too low.
💡 Tip: Monitor screen-out rates. If more participants are screened out than expected, your incidence estimate may have been too high.