The log rank test is based on a comparison of the observed and expected numbers of death, with the expected number calculated under the assumption of no difference in survival between groups. E_A and E_B are calculated as shown in the previous slide. O_A and O_B are the actual, observed numbers of deaths in group A and group B. The log rank test statistics is compared with the critical values of a Chi-square distribution. The larger the discrepancies between the observed and expected number of deaths in the groups, the more likely the log rank test statistics will be larger than the critical value of a Chi-square distribution with 1 degree of freedom (in the case of two groups); and therefore more likely to reject the null hypothesis of no difference.