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From |
Steven Joel Hirsch Samuels <sjhsamuels@earthlink.net> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Interpretation of sts list |

Date |
Wed, 14 Nov 2007 22:21:56 -0500 |

To add to Svend's explanation: Your calculation 17/48 assumes that all 48 people were at risk for the full study period--to 39 days. Not all were. For example, 2 of 48 failed between 1 and 5 days, but only 42 were at risk at 5 days. That means that 4 people (48 - 2 -42) had observation end during the period while still alive. These are referred to as 'censored'. So, you cannot use all 48 as the denominator for failure. Call Duke's Dept of Biostatistics and Informatics, and I am sure someone will take you through the basic calculations.. A couple of online resources are:

http://socserv.mcmaster.ca/jfox/Courses/soc761/survival-analysis.pdf

http://www.statsoft.com/textbook/glosc.html#Censoring

The topic is covered, as Sven states, in almost every biostatistics book.

On Nov 14, 2007, at 4:16 PM, Christina M Gibson-Davis wrote:

Hello,

I am having difficulty interpreting the "survivor function" column of sts list. I had thought it reported the percentage of cases that had survived during a particular time interval, and thus it would be possible to tell, by looking at the last entry, how many cases had survived without failing. However, by using the cancer data available through Stata (webuse cancer), I see that this cannot be.

If I type:

webuse cancer

stset study died

stdes

I see that there are 48 subjects, and 31 failed, for a survival rate of 35% (48-31=17; 17/48 = 35%).

But if I type "sts list, at(0 5 to 39)", I end up with a survival rate of 19% (please see below).

Beg. Survivor Std.

Time Total Fail Function Error [95% Conf. Int.]

---------------------------------------------------------------------- ---------

1 48 2 0.9583 0.0288 0.8435 0.9894

5 42 6 0.8333 0.0538 0.6943 0.9129

9 32 6 0.7061 0.0661 0.5546 0.8143

13 24 6 0.5628 0.0742 0.4060 0.6931

17 20 3 0.4870 0.0761 0.3326 0.6249

21 16 0 0.4870 0.0761 0.3326 0.6249

25 10 6 0.2922 0.0767 0.1543 0.4449

29 8 1 0.2557 0.0753 0.1247 0.4093

33 4 1 0.1918 0.0791 0.0676 0.3634

37 2 0 0.1918 0.0791 0.0676 0.3634

---------------------------------------------------------------------- ---------

I don't understand why the survivor function doesn't end up at 35%, if overall, 35% of the cases survived.

Thank you very much in advance for your assistance.

Christina

___________________________________________

Christina Gibson-Davis

Assistant Professor of Public Policy

Center for Child and Family Policy

Terry Sanford Institute

Duke University

Box 90245

Durham NC 27708

cgibson@duke.edu

(919) 613-7364

The thing that impresses me the most about America is the way parents obey

their children.

-- King Edward VIII

__________________________________________

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Steven Samuels sjhsamuels@earthlink.net 18 Cantine's Island Saugerties, NY 12477 Phone: 845-246-0774 EFax: 208-498-7441 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Interpretation of sts list***From:*Christina M Gibson-Davis <cgibson@duke.edu>

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