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

_{eff} denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:
:$1~\backslash text\; =\; \backslash rho\; \backslash cdot\; 10^5$
This unit is commonly used in the operation of

epidemiology
Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population.
It is a cornerstone of public health, and shapes policy decisions and evidenc ...

, and in nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from n ...

engineering as a unit of reactivity.
Epidemiology

Statistics
Statistics (from German: ''Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industri ...

of crime rates
Crime statistics refer to systematic, quantitative results about crime, as opposed to crime news or anecdotes. Notably, crime statistics can be the result of two rather different processes:
* scientific research, such as criminological studies, vi ...

, mortality and disease prevalence in a population are often given in
Nuclear Reactivity

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: :$\backslash rho=(k\_-1)/k\_$ where klight-water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron react ...

sites because reactivity values tend to be small, so measuring in pcm allows reactivity to be expressed using whole numbers.
Related units

*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, but a 10-percent increase in the quantity being me ...

difference of 1 part in 100
* Percentage
In mathematics, a percentage (from la, per centum, "by a hundred") 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 u ...

(%) 1 part in 100
* Per mille
Per mille (from Latin , "in each thousand") is an expression that means parts per thousand. Other recognised spellings include per mil, per mill, permil, permill, or permille.
The associated sign is written , which looks like a percent sig ...

(‰) 1 part in 1,000
* Basis point
A basis point (often abbreviated as bp, often pronounced as "bip" or "beep") is one hundredth of 1 percentage point. The related term ''permyriad'' means one hundredth of 1 percent. Changes of interest rates are often stated in basis points. If ...

(bp) difference of 1 part in 10,000
* Permyriad
A basis point (often abbreviated as bp, often pronounced as "bip" or "beep") is one hundredth of 1 percentage point. The related term '' permyriad'' means one hundredth of 1 percent. Changes of interest rates are often stated in basis points. If ...

(‱) 1 part in 10,000
* Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

including parts-per million, parts-per billion etc
See also

*InHour InHour is a unit of reactivity of a nuclear reactor. It stands for the inverse of an hour. It is equal to the inverse of the period in hours. One InHour is the amount of reactivity needed to increase the reaction from critical to where the power ...

(another unit of reactivity)
* Dollar (reactivity)
A dollar is a unit of reactivity for a nuclear reactor, calibrated to the interval between the conditions of delayed criticality and prompt criticality. Zero dollars is defined to be the threshold of slow criticality, which means a steady reactio ...

* Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo-units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they ...

* Per-unit system In the power systems analysis field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not chang ...

* Percent point function
In probability and statistics, the quantile function, associated with a probability distribution of a random variable, specifies the value of the random variable such that the probability of the variable being less than or equal to that value eq ...

Notes

Units of measurement {{engineering-stub