Number needed to screen

The number needed to screen is a way of summarizing the benefit of a screening program in health care. The calculations are derived from the results of a randomized controlled trial of a screening program.

Two-by-two table for a screening program

		Outcome
		Present	Absent
Screening	Screen group	Cell A	Cell B	Total in screening group
	Control group	Cell C	Cell D	Total in control group
		Total with outcome	Total without outcome

Calculations

Event rates

${\mbox{Experimental event rate}}=\left({\frac {\mbox{Cell A}}{\mbox{Cell A + Cell B}}}\right)$

${\mbox{Control event rate}}=\left({\frac {\mbox{Cell C}}{\mbox{Cell C + CellD}}}\right)$

Measures of efficacy

${\mbox{Absolute risk reduction}}=\left({\mbox{Experimental event rate}}-{\mbox{Control event rate}}\right)$

${\mbox{Number need to screen}}=\left({\frac {100}{\mbox{Absolute risk reduction}}}\right)$

See also

Mass screening