GraphPad Statistics Guide

Q & A: Entering survival data

Q & A: Entering survival data

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Q & A: Entering survival data

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How do I enter data for subjects still alive at the end of the study?

Those subjects are said to be censored. You know how long they survived so far, but don't know what will happen later. X is the # of days (or months…) they were followed. Y is the code for censored observations, usually zero.

What if two or more subjects died at the same time?

Each subject must be entered on a separate row. Enter the same X value on two (or more) rows.

How do I enter data for a subject who died of an unrelated cause?

Different investigators handle this differently. Some treat a death as a death, no matter what the cause. Others treat death of an unrelated cause to be a censored observation. Ideally, this decision should be made in the study design. If the study design is ambiguous, you should decide how to handle these data before unblinding the study.

Do the X values have to be entered in order?

No. You can enter the rows of data in any order you want. It just matters that each Y value (code) be on the same row as the appropriate X value.

How does Prism distinguish between subjects who are alive at the end of the study and those who dropped out of the study?

It doesn't. In either case, the observation is censored. You know the patient was alive and on the protocol for a certain period of time. After that you can't know (patient still alive), or can't use (patient stopped following the protocol) the information. Survival analysis calculations treat all censored subjects in the same way. Until the time of censoring, censored subjects contribute towards calculation of percent survival. After the time of censoring, they are essentially missing data.

I already have a life-table showing percent survival at various times. Can I enter this table into Prism?

No. Prism only can analyze survival data if you enter survival time for each subject. Prism cannot analyze data entered as a life table.

Can I enter a starting and ending date, rather than duration?

Yes. When you create a new survival table, you can choose to enter starting and ending dates, rather than number of days.  

How do I handle data for subjects that were “enrolled” but never treated?

Most clinical studies follow the “intention to treat” rule. You analyze the data assuming the subject got the treatment they were assigned to receive, even if the treatment was never given. This decision, of course, should be made as part of the experimental design.

If the subject died right after enrollment, should I enter the patient with X=0?

No. The time must exceed zero for all subjects. If you enter X=0, Prism simply ignores that row. More on survival curves with X=0.