The effective dose is used to compare the stochastic risk of non-uniform exposure to radiation. Body tissues react differently to radiation and cancer-induction occurs at different rate of dose in different tissues. Hence, the effective dose is the risk of developing fatal cancer in the tissue in question. If the body is uniformly irradiated, the summed effective doses are equal to 1.
For any examination, the effective dose to each body part can be summed after being multiplied by its weighting factor.