Which statement about rates is correct?

• A. A plain ratio without time
• B. Proportion is always used
• C. The number of cases in question end
• D. A ratio with a distinct relationship between numerator and denominator and a measure of time is an intrinsic part of the denominator

Answer: (D)

Rates are frequencies that measure how often something happens in a defined amount of time, so time must be part of the denominator. That means you’re looking at a defined relationship between the number of events (the numerator) and the time at risk (the denominator), such as cases per person-time. This is what makes a rate different from a plain ratio or a simple count: the time dimension is essential.

The best statement captures this by describing a ratio in which the numerator and denominator are linked by a defined relationship and includes a time measure in the denominator, for example events per person-time. A plain ratio without time isn’t a rate, since it lacks the time component. Proportions aren’t rates either because they don’t incorporate time in the denominator. And a description that treats the count alone or without a time basis doesn’t reflect the per-time relationship that defines a rate.