Dear Theo,
the following one is a temptative answer to your message.
I assume I have 20 observations ordered in ranks (degrees of freedom (d_f) are 20-20=18, according to the hypothesis test formula for Spearman's rank correlation coefficient).
Ho to be tested is rho=0.
I have calculated a random Spearman's ranK correlation coefficient, t (ts) and then the p-value.
....................begin example......................
set obs 1
g r_Spearman=1*uniform()
g d_f=(20-2)
g ts= r_Spearman*[(20-2)/(1-(-r_Spearman^2))]^.5
g p_value=ttail(d_f, ts)
...................end example.......................
HTH and Kind Regards,
Carlo
-----Messaggio originale-----
Da: [email protected] [mailto:[email protected]] Per conto di Theo Serge
Inviato: giovedì 19 giugno 2008 8.24
A: [email protected]
Oggetto: st: Spearman's rank correlation coefficient - power calculation
Dear Statalist,
I would be grateful if you could help me on a problem:
Having submitted an article to a Journal, the reviewers asked me for power calculation on a Spearmanʼs rank correlation coefficient.
This happened although I had been very careful with the interpretation of negative findings in the manuscript. Moreover, Spearmanʼs rank correlation coefficient was the appropriate choice for the univariate analysis performed, due to a variety of underlying factors.
To my knowledge, there is no command through which STATA can perform power calculations on Spearmanʼs rank correlation coefficients.
Please inform me if there is a way to overcome this difficulty. Are there any commands I am not familiar with? Would programming STATA be of value?
What would be the optimal approach to this issue? The need for power calculations in Spearman's rank correlation coefficients seems justified.
I thank you in advance.
T.S.
University of Athens, Greece
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