Incidence rate: the formula, and how it gets misread
The arithmetic takes one line. The disagreements come from what people put in the denominator, and nobody writes that part down.
Outline. Each heading below carries one line saying what the section has to establish. Expand each into two to four paragraphs, put at least one number or worked example in the piece that nobody else could publish, then delete this note.
Incidence rate is the first number on every feasibility conversation and the last one anybody agrees on. The formula is trivial; the argument is always about what went underneath the line.
The formula, and the one line of arithmetic
State IR = qualified / screened, then show the same fieldwork producing two different rates under two denominators.
Four denominators that all call themselves incidence
Panel base, profiled base, entered-the-screener, completed-the-screener. Give the typical spread between them.
Why the quoted number is almost always the flattering one
Not dishonesty — the profiled base is what a supplier can compute without fielding. Say when that is legitimate.
Drop-off is not a screen-out, and mixing them inflates IR
Explain the difference and the size of the error it introduces on long screeners.
What to ask before you sign
One question: what did you divide by. A supplier who cannot answer immediately did not build the report.
Recording the real rate so the next quote is better
Observed IR per study is the calibration data that makes later estimates honest. Most buyers never send it back.
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Healthcare sample and the cost of verification
Self-reported clinical roles are wrong often enough that the whole category is priced around finding out.
9 min read -
Writing a screener people cannot game
Most screeners announce what they are looking for in the first two questions, and then the researcher is surprised that everyone qualifies.
6 min read -
Your B2B incidence rate is probably wrong
Two people can compute incidence from the same fieldwork and disagree by a factor of two, both correctly. The disagreement is almost never about arithmetic.
5 min read
