Sample size or precision for a mean difference

prec_meandiff returns the sample size or the precision for the provided mean difference and standard deviations. For paired differences, use prec_mean, as it is equivalent to a simple mean.

prec_meandiff(
  delta,
  sd1,
  sd2 = sd1,
  n1 = NULL,
  r = 1,
  conf.width = NULL,
  conf.level = 0.95,
  variance = c("equal", "unequal"),
  ...
)

Arguments

delta

difference in means between the two groups.

sd1

standard deviation in group 1.

sd2

standard deviation in group 2.

n1

number of patients in group 1.

r

allocation ratio (relative size of group 2 and group 1 (n2 / n1)).

conf.width

precision (the full width of the confidence interval).

conf.level

confidence level.

variance

equal (default) or unequal variance.

...

other options to uniroot (e.g. tol)

Value

Object of class "presize", a list of arguments (including the computed one) augmented with method and note elements.

Details

Exactly one of the parameters n or conf.width must be passed as NULL, and that parameter is determined from the other.

Examples

# mean difference of 5, SD of 2.5, CI width with 20 participants assuming equal variances
prec_meandiff(delta = 5, sd1 = 2.5, n1 = 20, var = "equal")
#> 
#>      precision for mean difference with equal variance 
#> 
#>   delta sd1 sd2 n1 n2 conf.width conf.level      lwr      upr
#> 1     5 2.5 2.5 20 20   3.044614       0.95 3.477693 6.522307
# mean difference of 5, SD of 2.5, number of participants for a CI width of 3,
#  assuming equal variances
prec_meandiff(delta = 5, sd1 = 2.5, conf.width = 3, var = "equal")
#> 
#>      sample size for mean difference with equal variance 
#> 
#>   delta sd1 sd2       n1       n2 conf.width conf.level lwr upr
#> 1     5 2.5 2.5 20.61861 20.61861          3       0.95 3.5 6.5