title: Per cent mille
text: # Per cent mille
  
  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.
  
  ## Epidemiology
  
  Statistics of crime rates, mortality and disease prevalence in a population are often given in "per 100 000".
  
  ## 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:
  
      $\rho =(k_{\text{eff}}-1)/k_{\text{eff}}$
  
  where $k_{eff}$ denotes the effective multiplication factor for the reaction. Therefore, one pcm is equal to:
  
      $1~{\text{pcm}}=\rho \cdot 10^{5}$
  
  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.
  
  ## Related units
  
  * Percentage point difference of 1 part in 100
  * Percentage (%) 1 part in 100
  * Per mille (‰) 1 part in 1,000
  * Basis point (bp) difference of 1 part in 10,000
  * Permyriad (‱) 1 part in 10,000
  * Parts-per notation including parts-per million, parts-per billion etc
  