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Concept# Per cent mille

Summary

A per cent mille or pcm is one one-thousandth of a percent. It can be thought of as a "milli-percent". It is commonly used in epidemiology, and in nuclear reactor engineering as a unit of reactivity.
Statistics of crime rates, mortality and disease prevalence in a population are often given in "per 100 000".
In nuclear reactor engineering, a per cent mille is equal to one-thousandth of a percent of the reactivity, denoted by Greek lowercase letter rho. Reactivity is a dimensionless unit representing a departure from criticality, calculated by:
where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:
This unit is commonly used in the operation of light-water reactor sites because reactivity values tend to be small, so measuring in pcm allows reactivity to be expressed using whole numbers.

Official source

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Related concepts (3)

Percentage point

A percentage point or percent point is the unit for the arithmetic difference between two percentages. For example, moving up from 40 percent to 44 percent is an increase of 4 percentage points (although it is a 10-percent increase in the quantity being measured, if the total amount remains the same). In written text, the unit (the percentage point) is usually either written out, or abbreviated as pp or p.p. to avoid confusion with percentage increase or decrease in the actual quantity.

Per mille

Per mille () is parts per thousand. Other recognised spellings include per mil, per mill, permil, permill, or permille. The associated sign is written , which looks like a per cent sign with an extra zero or o in the divisor. Major dictionaries do not agree on the spelling and some dictionaries, such as Macmillan, do not even contain an entry. One common usage is blood alcohol content, which is usually expressed as a percentage in English-speaking countries. Per mille should not be confused with parts per million (ppm).

Percentage

In mathematics, a percentage () is a number or ratio expressed as a fraction of 100. It is often denoted using the percent sign (%), although the abbreviations pct., pct, and sometimes pc are also used. A percentage is a dimensionless number (pure number), primarily used for expressing proportions, but percent is nonetheless a unit of measurement in its orthography and usage. For example, 45% (read as "forty-five per cent") is equal to the fraction 45/100, the ratio 45:55 (or 45:100 when comparing to the total rather than the other portion), or 0.

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