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.