# Can I change number of people at risk during study in survival analysis?

I need to estimate the survival function using Kaplan-Meier method. I am considering the duration of 60 days. In the beginning (Day 1), lets say there are some 400 individuals. My question is: 'after my study has started, can I add more people into the study on day 20th. Does KM method allow this? Will my survival probabilities will be still correct after adding new subjects?

If you check the clinical trials, especially for cancer treatment, survival function is one of the objectives. In the clinical trials, it is nearly impossible to recruit enough patients at the short time. So the patients entered the study at the different time and each individual's survival time is used to estimate the survival function using K-M estimate. For each patient, the day 1 is defined as the day entering the study. But the assumption behind this is the survival probability at day $n$ are the same for the patents received the same treatment regardless when the patients entered study.