# How to calculate log-rank test statistic

Depending on which statistics text you read you will get 2 different formulations of the log-rank test statistic. In some texts you will see it specified as:

$$\frac{(O-E)^2}{E}$$ Example 1

Whilst in others you will see it as $$\frac{(O-E)^2}{V}$$

My questions are:

• Do these variants have specific names?
• Do they have any differences in properties?
• When should one be used over the other?