Understanding the Manual Log Rank Test
The log rank test compares survival experience between two groups. It uses event times, risk sets, and observed events. It does not compare average survival directly. Instead, it checks whether the event pattern differs across the whole follow up period.
Why the Test Matters
Survival studies often include censored cases. A censored case has not shown the event by its last known time. The log rank method handles this by using the number still at risk before each event time. This makes the test useful in medical, reliability, product life, and retention studies.
Manual Table Method
A manual table starts with each distinct event time. For every row, enter the number at risk in group A and group B. Then enter the observed events for both groups. The calculator finds the total at risk and total events. It then assigns the expected events for group A by proportional risk. Group B is the remaining expected count.
Variance and Test Statistic
The variance measures how much the observed difference can vary by chance. Each row adds a variance term. The final chi square value uses the squared observed minus expected difference. A larger value means a larger separation between curves. The p value is based on one degree of freedom.
Reading the Result
When the p value is below alpha, the curves differ statistically. This does not prove a clinical effect alone. Check sample size, censoring pattern, study design, and endpoint definition. Also inspect survival curves when available. The test assumes comparable censoring and proportional hazards over time.
Good Data Practice
Use one row for each event time. Combine tied events into the same row. Do not list censored observations as events. Adjust the risk set after censored cases for later rows. Keep the event meaning consistent. For example, death, relapse, churn, or device failure should not be mixed without planning.
Practical Use
This calculator is designed for manual checking. It shows expected counts, variance, row contributions, and final significance. The CSV and PDF outputs help preserve audit notes. They are helpful for assignments, study reports, and quick verification work. Document every edited row clearly. Record sources before final reporting. Keep reviews simple and traceable.