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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 Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in defined populations. It is a cornerstone of public health, and shapes policy decisions ...
, and in
nuclear reactor A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a fission nuclear chain reaction 300px, A possible nuclear fission chain reaction: 1) A uranium-235 atom absorbs a neutron">uranium-235.html" ;"ti ...

engineering as a unit of reactivity.

# Epidemiology

Statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data Data (; ) are individual facts, statistics, or items of information, often numeric. In a more technical sens ...

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, vict ...
,
mortality Mortality is the state of being mortal, or susceptible to death (1906) Death is the permanent, Irreversible process, irreversible cessation of all biological process, biological functions that sustain a living organism. Brain death is some ...
and 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 Rho (uppercase Ρ, lowercase ρ or ; el, ῥῶ) is the 17th letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late ninth or early eighth century BC. It is derived from the earlier Phoenician ...
. Reactivity is a dimensionless unit representing a departure from criticality, calculated by: :$\rho=\left(k_-1\right)/k_$ where keff denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to: :$1~\text = \rho \cdot 10^5$ This unit is commonly used in the operation of
light-water reactor The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to Pressurized heavy-water reactor, heavy water, as both its coolant and neutron moderator – furthermore a solid form of fissile elements is us ...
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 Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit ...
difference of 1 part in 100 *
Percentage In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...

(%) 1 part in 100 *
Per mille Per mille (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of ...
(‰) 1 part in 1,000 *
Basis point A basis point (often abbreviated as bp, often pronounced as "bip" or "beep") is (a difference of) one hundredth of a percent or equivalently one percent of one percent or one ten thousandth. A very rarely used term, permyriad, means parts per te ...
(bp) difference of 1 part in 10,000 *
Permyriad A basis point (often abbreviated as bp, often pronounced as "bip" or "beep") is (a difference of) one hundredth of a percent or equivalently one percent of one percent or one ten thousandth. A very rarely used term, permyriad, means parts per te ...
(‱) 1 part in 10,000 *
Parts-per notation In science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictio ...
including parts-per million, parts-per billion etc

* InHour (another unit of reactivity) * Dollar (reactivity) *
Parts-per notation In science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable explanations and predictio ...
*
Per-unit systemIn 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 Probability is the branch of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculu ...

# Notes

Units of measurement {{engineering-stub