Extrapolation of correlation between 2 variables in 4 general medical journals.

CONTEXT An estimated correlation between 2 variables is valid only within the range of observed data. Extrapolation is risky and should be performed with caution. METHODS To assess the prevalence of problems with data extrapolation in the medical literature, all articles published from January through June 2000 in BMJ, JAMA, The Lancet, and The New England Journal of Medicine (NEJM) were reviewed manually. Articles containing at least 1 scatterplot with raw data and a corresponding fitted regression line were included in the analysis. Articles were considered to involve extrapolation if they contained at least 1 fitted line beyond the observed data in any scatter plot. RESULTS A total of 178 articles presenting at least 1 scatterplot were identified. Among them, 37 articles (21%) (5 from BMJ, 7 from JAMA, 23 from The Lancet, and 2 from NEJM) were included. Twenty-two articles (59% [95% confidence interval, 42%-75%]) from all 4 journals involved extrapolation. None changed the line type to indicate extrapolation. Four articles (11%) contained a plot in which the fitted line reached unreasonable or meaningless values. Three articles (8%) stated explicit conclusions about values outside the range of the observed data. CONCLUSIONS A high proportion of the articles analyzed from all 4 weekly general medical journals involved extrapolation without indication. Researchers, reviewers, and editors should be aware of this problem and work to eliminate it.