Running a Nelson-Aalen analysis with XLSTAT-Life?
An Excel sheet with both the data and results can be downloaded by clicking here.
The data have been obtained in [Gehan E.A. (1965). A generalized Wilcoxon test for comparing arbitrarily singly-censored samples. Biometrika, 52, pp 203—223] and represent a randomized clinical trial investigating the effect of the drug 6-mercaptopurine on remission times (in weeks) of acute leukemia patients.
Our goal is to determine if and how the drug influences the survival time, by comparing the hazard curves for two groups of 21 patients, the first being treated, and the second being a control group. All 21 patients of the control group were observed to have a recurrence of their leukemia. Only 9 of the 6-MP patients had an observed recurrence time, while the 12 others were censored.
Nelson-Aalen estimation of cumulative hazard function should be preferred to Kaplan-Meier estimator when the user is interested in hazard functions.
After opening XLSTAT, select the XLSTAT/XLSTAT-Life/Nelson-Aalen analysis command, or click on the corresponding button of the "XLSTAT-Life" toolbar (see below).
Once you've clicked on the button, the Nelson-Aalen analysis box will appear. Select the data on the Excel sheet. The "Time data" corresponds to the durations when the patients either relapsed or were censored. The "Status indicator" describes whether a patient relapsed (event code=1) or was censored (censored code = 0) at a given time. So that XLSTAT takes into account the information whether the patient belongs to the control or the treated group, we need to select the groups information, and to activate the "compare" option so that the comparison tests are computed.
The computations begin once you have clicked on "OK". The results will then be displayed on a new Excel sheet.
Interpreting the results of a Nelson-Aalen analysis
The results for the first group are displayed first. The first table displays a summary of the data for the "6-MP drug" patients.
The next table corresponds to the "Nelson-Aalen table". It contains the results of the Nelson-Aalen analysis with several key indicators.
Then, we can visualize several curves, including the cumulative hazard function, bounded by the confidence intervals. The circles identify the censored data.
Next, the same series of results is displayed for the control group.
Then, we can compare the two groups. First, a series of tests is displayed in a table. From the results we can see that the difference between the two survivor functions is very significant.
Last the comparison of the two hazard curves allows us to conclude to confirm that the drug impacts significantly negatively the hazard to which the patients are exposed.
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