size.ci.slope {statpsych}

Sample size for a slope confidence interval

Description

Computes the total sample size required to estimate a 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.

Usage

size.ci.slope(alpha, evar, x, w)

Arguments

alpha

alpha level for 1-alpha confidence

evar

planning value of within group (error) variance

x

vector of x values of the quantitative factor

w

desired confidence interval width

Value

Returns the required total sample size

Examples

x <- c(2, 5, 8)
size.ci.slope(.05, 31.1, x, 1)

# Should return:
# Total sample size
#                83