pi.prop: Prediction interval for an estimated proportion

In statpsych: Statistical Methods for Psychologists

Prediction interval for an estimated proportion

Description

Computes approximate one-sided or two-sided prediction interval for the estimated proportion in a future study with a planned sample size of n. The prediction interval uses a proportion estimate from a prior study that had a sample size of n0.

Several confidence interval sample size functions in this package require a planning value of the estimated proportion that is expected in the planned study. A one-sided proportion prediction limit is useful as a proportion planning value for the sample size required to obtain a confidence interval with desired width. This strategy for specifying a proportion planning value is useful in applications where the population proportion in the prior study is assumed to be very similar to the population proportion in the planned study.

For sample size planning, use an upper prediction limit if the population proportion is assumed to be less than .5. If the upper prediction limit is greater than .5, then set the proportion planning value to .5. Use a lower prediction limit if the population proportion is asumed to be greater than .5. If the lower prediction limit is less than .5, then set the proportion planning value to .5.

Usage

pi.prop(alpha, prop, n0, n, type)

Arguments

alpha	alpha value for 1-alpha confidence
prop	estimated proportion from prior study
n0	sample size used to estimate proportion in prior study
n	planned sample size of future study
type	set to 1 for two-sided prediction interval set to 2 for one-sided upper prediction limit set to 3 for one-sided lower prediction limit

Value

Returns a prediction interval for an estimated proportion in a future study

Examples

pi.prop(.1, .225, 80, 120, 1)

# Should return:
#         LL       UL
#  0.1390955 0.337095
 

statpsych documentation built on April 4, 2025, 3:20 a.m.