The Kaplan-Meier method produces survival curves. To test the null hypothesis that two survival curves are identical, e.g. in a clinical trial with a control group (group A) and a treatment group (group B), the log rank test is commonly used. In the slide we use the subscript i to indicate "at time i", when a death is observed. The number of expected deaths in group A at time i (E_Ai) is equal to the number of at risk subjects in group A at time i divided by the total (groups A & B) number of at risk subjects at time i, multiplied by the total number of deaths at time i. E_Bi is calculated in the same way. Total expected deaths in group A and B (E_A & E_B) are then calculated by summing up the expected numbers at different times (i.e., sum E_Ai & sum E_Bi).