Missing confidence interval for 25th percentile

[Margaret MacDougall]

Dear all

  I would be most grateful for some explanation as to why on some occasions the output for a Kaplan-Meier analysis can list the value for the 25th percentile of the survival time distribution (under '75.00') but not the confidence interval.  Does this problem have something to do with the use of Greenwood's formula in calculating the standard error for this percentile? In fact, what formula does SPSS use to obtain the standard error?

  Many thanks for your input.

  Best wishes

  Margaret



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[Marta García-Granero]

Hi Margaret

MM>   I would be most grateful for some explanation as to why on
MM> some occasions the output for a Kaplan-Meier analysis can list the
MM> value for the 25th percentile of the survival time distribution
MM> (under '75.00') but not the confidence interval.  Does this
MM> problem have something to do with the use of Greenwood's formula
MM> in calculating the standard error for this percentile?

I think that the formula needs to have survival values at both sides
of the percentile.

MM> In fact, what formula does SPSS use to obtain the standard error?

You can get that info from the installation CD, folder "Algorithms"
file KM.pdf (if you don't have access to those files you can also
download them from:

http://support.spss.com/Tech/Products/SPSS/Documentation/Statistics/algorithms/index.html

If the link is broken in two lines copy-paste them together
If requested to login use "Guest" both as User and password


--
Regards,
Dr. Marta García-Granero,PhD           mailto:[hidden email]
Statistician

---
"It is unwise to use a statistical procedure whose use one does
not understand. SPSS syntax guide cannot supply this knowledge, and it
is certainly no substitute for the basic understanding of statistics
and statistical thinking that is essential for the wise choice of
methods and the correct interpretation of their results".

(Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)

[Margaret MacDougall]

Dear Marta

  Thank you for this information.  I have studied the file and see that it is indeed Greenwood's formula which has been assumed and that an increment of 0.05 is used in obtaining each confidence limit.


  On examining the survival table, I was initially rather puzzled why CIs were not been provided where they were calculable but on closer examination, I have observed that if a confidence interval is not forthcoming for one value of the factor variable it is automatically not calculated for the other value.

  Best wishes

  Margaret

Marta García-Granero <[hidden email]> wrote:
  Hi Margaret

MM> I would be most grateful for some explanation as to why on
MM> some occasions the output for a Kaplan-Meier analysis can list the
MM> value for the 25th percentile of the survival time distribution
MM> (under '75.00') but not the confidence interval. Does this
MM> problem have something to do with the use of Greenwood's formula
MM> in calculating the standard error for this percentile?

I think that the formula needs to have survival values at both sides
of the percentile.

MM> In fact, what formula does SPSS use to obtain the standard error?

You can get that info from the installation CD, folder "Algorithms"
file KM.pdf (if you don't have access to those files you can also
download them from:

http://support.spss.com/Tech/Products/SPSS/Documentation/Statistics/algorithms/index.html

If the link is broken in two lines copy-paste them together
If requested to login use "Guest" both as User and password


--
Regards,
Dr. Marta García-Granero,PhD mailto:[hidden email]
Statistician

---
"It is unwise to use a statistical procedure whose use one does
not understand. SPSS syntax guide cannot supply this knowledge, and it
is certainly no substitute for the basic understanding of statistics
and statistical thinking that is essential for the wise choice of
methods and the correct interpretation of their results".

(Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)



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