This chapter deals with statistical characteristics of radioactive decay, covering the issues of error, accuracy and precision. Measurement of a quantity is characterized by an average or mean and its standard deviation, the latter showing the spread of deviations of the measurements from the average value. Propagation of errors in addition, subtraction, multiplication, and division is illustrated. Random nature of radioactive decay follows Poisson distribution which is briefly discussed. Gaussian distribution and different confidence levels in measurements are presented. Also, a section on the assessment of sensitivity, specificity, and predictive values in nuclear diagnostic tests is presented. Application of chi-square test is explained with an example.