Data in the MMWR
Description (3 of 12)
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Data in the MMWR (§5.2) On this site (opens in a new tab)
Describe quantities, including outcomes
Principles: Descriptive quantities, such as mean, proportion, rate, ratio, variance, range, and standard error, are properly named. Descriptive quantities are formatted with sufficient precision—the number of significant digits and number of decimal places—to offer meaningful distinctions but no more than supported by the resolution of the data. Format precision is consistent across quantities of similar type.
Observations: In some cases, the word “rate” is used for a proportion. One example reports the number of deaths per 100,000 population, standardized to the 2000 Census population, as “death rates”; the authors might be implicitly treating the unit as 100,000 person-years. In contrast, another report presents standardized mortality rates as the number of deaths per 100 person-years. Some reports use as many as 9 significant digits, typically for count values in large datasets or populations. Some reports have large variation in the number of significant digits.
Recommendations: Ensure consistent use of the terms proportion, rate, and ratio, each of which pertains to the result of dividing one number (the numerator) by another (the denominator).
Proportion: The numerator and denominator have the same units, and both are typically nonnegative integers. A proportion is unitless and typically expressed as a fraction or percentage.
Rate: The numerator and denominator have different units, both are typically nonnegative, and the denominator unit typically incorporates time or space. A rate is not unitless, e.g., X incident cases per person-year.
Ratio: See the next topic.
Consider both the number of decimal places when rounding and the number of significant digits. In general, we recommend preserving 2-4 significant digits throughout a report. The number of decimal places, typically 0-2, should be chosen in a way that generally preserves the intended number of significant digits.
Next topic: Compare quantities: differences and ratios (§5.3)
Data in the MMWR
- Introduction · How learners read
- 1 · Sources
- 2 · Methods
- 3 · Description (this post)
- 4 · Comparison
- 5 · Adjustment
- 6 · Conclusions
- 7 · Tables
- 8 · Figures
- 9 · Interpretation
- 10 · Impact
- 11 · Context
- 12 · Additional practices