Almost an extract from Statistical Questions in Evidence-based Medicine by Martin Bland and Janet Peacock.
This question is a bonus, as it was not actually included in Statistical Questions in Evidence-based Medicine, due to lack of space. We hope that the topic will be useful in own right, as well as giving a flavour of the book.
| Age |
|Years of education|| |
The relationship between age and years of education was analysed using Spearman's rank correlation coefficient, rho = -0.45, P<0.0001. It is the category variables which are being correlated not individual exact ages or years of education. These may be very difficult to ascertain in this population (Gupta et al. 1994).
+ 1. Why would a rank correlation be preferred
to the product moment correlation coefficient?
Check answer 1
+ 2. Do you think Spearman's is the best choice
Check answer 2
The patients were given a clinical examination and systolic and diastolic blood pressure and lipoprotein lipid concentrations were obtained. The relationships of these to educational level were tested using Kendall's tau. The following rank correlations, Spearman's and Kendall's, between blood pressure and educational level were quoted, all significant (P<0.001):
+ 3. Does it make any difference which rank
correlation is used? Is there any advantage in quoting both?
Check answer 3
+ ! 4. Does the fact that the Kendall's tau is
less than Spearman's in three of the four cases tell us anything?
Check answer 4
Reference: Gupta, R, Gupta, VP, Ahluwalia, NS. (1994) Educational status,
coronary heart disease, and coronary risk factor prevalence in a rural
population of India. British Medical Journal 309 1332-1336.
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