Medical statistics are often reported in either too many or too little decimal places says a professor of medical statistics who urges researchers to adopt a ‘rule of four’. 

This new yet evidence based reporting rule ensures that the rounding error is less than 1.3% for all ratios, whatever their value, says Professor Tim Cole from the UCL Institute of Child Health in The BMJ. 

The rule states: “Round the risk ratio to two significant digits if the leading non-zero digit is four or more, otherwise round to three;” it uses three decimal places for ratios in the range 0.040-0.399, two decimals for 0.40-3.99, one decimal for 4.0-39.9, and so on.

Professor Cole says his audit of 1250 risk ratios and confidence intervals from articles published in the journal in 2011-13 shows that just seven were reported to three decimal places, that 312 (25%) would be reported to other than two decimal places if the rule of four were used, and that it would reduce the mean fractional error from 0.8% to 0.5% (P<0.0001).