Description
Return fraction = events/exposedPersons when exposedPersons is positive, otherwise fraction = 0 and defined = false.
A cohort event proportion counts events relative to a defined group. Its interpretation as risk requires an appropriate initially at-risk cohort, a common observation period and compatible counting rules; an undefined denominator must remain distinguishable from a measured zero.
Return fraction = events/exposedPersons when exposedPersons is positive, otherwise fraction = 0 and defined = false.
When to use this tool
Use Cohort Event Exposure Fraction when summarizing a supplied cohort's event count with an explicit denominator.
How to use the tool and how it works
- Enter events, exposedPersons. Use the units and conventions below.
- Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
- Read fraction, defined. Inspect defined before interpreting the associated values.
Require nonnegative whole exposedPersons and whole events between zero and exposedPersons. Return fraction = events/exposedPersons when exposedPersons is positive, otherwise fraction = 0 and defined = false. The fraction is dimensionless; multiply by 100 separately for percent.1
An incidence proportion relates new cases to an initially disease-free population during a specified interval. That interpretation requires suitable event counting and follow-up beyond the arithmetic checked here.2
| Input field | Meaning and units | Accepted input |
|---|---|---|
| events | Events | Required. Safe integer; minimum 0. |
| exposedPersons | Exposed Persons | Required. Safe integer; minimum 0. |
| Output field | Interpretation | Returned shape |
|---|---|---|
| fraction | Dimensionless ratio under the stated denominator convention; not multiplied by 100. | Required. Finite number. |
| defined | Whether the denominator is positive and the returned ratio is defined; inspect this before using a zero placeholder. | Required. Boolean. |
{
"input": {
"events": 12,
"exposedPersons": 100
},
"output": {
"fraction": 0.12,
"defined": true
}
}Limitations and assumptions
- Interpret this as a cohort event proportion only when events count distinct persons from the supplied denominator over a defined period. It does not account for person-time, confounding, follow-up loss or repeated events.
- Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
- Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.
Alternative or Complementary methods
Use person-time rates or survival methods when follow-up differs, and examine study design before interpreting a proportion causally.
References
-
Calculation and input contract: epidemiology/cohort-event-exposure-fraction — tool.io
-
Measures of risk: morbidity frequency measures — Centers for Disease Control and Prevention
Similar or alternative tools
- Case Component Match Count
Count matched entries only where both flags are true, knownCount where known is true, and unknownCount where known is false.
- Incidence Density Exposure Day Factor Tool
Incidence density from explicitly supplied person-time; no assumption that events equal affected people.