*EAR.*

See excess risk.

*ERR.*

See excess risk.

*Excess risk.*

Risk, whether it is relative or absolute, is often expressed in terms of the excess that it represents over expectations in the absence of exposure to a carcinogenic agent, radiation in this instance. *Excess relative risk (ERR)* is, therefore, merely the observed relative risk minus 1, the value expected in the absence of an effect of radiation. Similarly *excess absolute risk (EAR)* is the number of incident cases or deaths at a particular dose above the number that would be expected in the absence of a radiation-related increase.

*Exposure*

Technically, exposure is the amount of air ionized by radiant energy, specifically, the amount of electrical charge produced in 1 cc of air under condition of electron equilibrium. More commonly, and in the present context, it simply means the presence of an individual in a field of radiation.

*Geometric mean (abbreviated GM).*

The geometric average of a series of n positive numbers; it is equal to the nth root of their product. As an illustration, the geometric mean of the numbers 2 and 8 is the square root of 16 (their product) or 4. The GM is approximately the median (50th percentile) of a distribution of numbers whose logarithms are normally distributed.

*Geometric standard deviation (abbreviated GSD).*

The geometric standard deviation is the antilog of the standard deviation of the natural logarithms of a set of numbers. The GSD is usually used to describe the variation of a set of data that is positively skewed.

*GM.*

See geometric mean.

*Gray (abbreviated Gy).*

The SI unit of absorbed dose equal to 1 joule per kilogram, or 100 rad. The unit derives its name from the English biophysicist Louis Harold Gray.

*GSD.*

See geometric standard deviation.

*Gy.*

See Gray.

*Kriging.*

Kriging is defined as the process of estimating the value of a spatially distributed variable from adjacent values while considering their interdependence.

*Probability of causation.*

A number that expresses the probability that a given cancer, in a specific tissue has been caused by a previous exposure to a carcinogenic agent, such as ionizing radiation.

*rad.*

The unit of absorbed dose used prior to the introduction of the Gray. One rad is equal to 100 ergs per gram or 0.01 Gy.