Computes the total sample size required to estimate a population slope with desired confidence interval precision in a between-subjects design with a quantitative factor. In an experimental design, the total sample size would be allocated to the levels of the quantitative factor and it might be necessary to increase the total sample size to achieve equal sample sizes. Set the error variance planning value to the largest value within a plausible range for a conservatively large sample size.
size.ci.slope(alpha, evar, x, w)alpha level for 1-alpha confidence
planning value of within group (error) variance
vector of x values of the quantitative factor
desired confidence interval width
Returns the required total sample size
x <- c(2, 5, 8)
size.ci.slope(.05, 31.1, x, 1)
#> Total sample size
#> 83
# Should return:
# Total sample size
# 83