Sample size or precision for a rate ratio

prec_rateratio returns the sample size or the precision for the provided proportions.

Usage

prec_rateratio(
  n1 = NULL,
  rate1 = NULL,
  rate2 = 2 * rate1,
  prec.level = NULL,
  r = 1,
  conf.level = 0.95
)

Arguments

n1

number of patients in exposed group.

rate1

event rate in the exposed group.

rate2

event rate in the unexposed group.

prec.level

ratio of the upper limit over the lower limit of the rate ratio confidence interval.

r

allocation ratio (relative size of unexposed and exposed cohort (n2 / n1)).

conf.level

confidence level.

Details

Exactly one of the parameters n1 or conf.width must be passed as NULL, and that parameter is determined from the other. Event rates in the two groups should also be provided (rate1, rate2). If only rate1 is provided, rate2 is assumed to be 2 times rate1.

References

Rothman KJ, Greenland S (2018). Planning Study Size Based on Precision Rather Than Power. Epidemiology, 29:599-603. doi:10.1097/EDE.0000000000000876 .

Examples

# 20 participants, a rate of 50%  against a rate of 300\%
prec_rateratio(20, .5, 3)
#> 
#>      precision for a rate ratio 
#> 
#>   n1 n2 r ntot rate1 rate2        rr        lwr       upr prec.level conf.level
#> 1 20 20 1   40   0.5     3 0.1666667 0.08533124 0.3255288   3.814884       0.95
# sample size required to attain a CI whose upper limit is not more than 3.81 larger
#  than the lower limit
prec_rateratio(rate1 = .5, rate2 = 3, prec.level = 3.81)
#> 
#>      sample size for a rate ratio 
#> 
#>         n1       n2 r     ntot rate1 rate2        rr        lwr       upr
#> 1 20.03833 20.03833 1 40.07666   0.5     3 0.1666667 0.08538592 0.3253204
#>   prec.level conf.level
#> 1       3.81       0.95