Spearman’s rank correlation coefficientBMJ 2014; 349 doi: https://doi.org/10.1136/bmj.g7327 (Published 28 November 2014) Cite this as: BMJ 2014;349:g7327
- Philip Sedgwick, reader in medical statistics and medical education
- Institute for Medical and Biomedical Education, St. George's, University of London, UK
- Correspondence to P Sedgwick
Researchers investigated the association between suicide rates and antidepressant prescribing in Australia.1 A retrospective analysis of national databases between 1991 and 2000 was undertaken. Participants were aged 15 years or more. Rates of suicide and antidepressant prescribing were recorded by sex and 10 year age groups (15-24, 25-34, 35-44, 45-54, 55-64, 65-74, 75-84, and ≥85 years). Suicide rates were expressed as the number per 100 000 population for each of the five-year periods of 1986-90 and 1996-2000. The changes in suicide rates from 1986 to 90 to 1996-2000 were derived. Antidepressant prescribing was expressed as the estimated defined daily doses per 1000 population per day (DDD/1000/day). Defined daily dose was based on the assumed average daily dose of the drug when used by adults for its main indication. The change in antidepressant prescribing from 1991 to 2000 was derived.
The researchers reported that from 1986 to 90 to 1996-2000, suicide rates decreased in older men and older women, and increased in younger men and younger women. From 1991 to 2000, rates of antidepressant prescribing increased across all age groups in both men and women. The changes in suicide rates were plotted against those in antidepressant prescribing across age groups, with men and women investigated separately (fig 1). Spearman’s rank correlation coefficient was used to measure the association between changes in suicide rates and antidepressant prescribing. For men and women, there was a negative correlation, with the largest declines in suicide in the age groups associated with the greatest increases in antidepressant prescribing. The association was significant in women (rs=−0.74; P=0.04) but not in men (rs=−0.62; P=0.10).