qnormGC {tigerstats}

Graphical Calculator for Normal Curve Percentiles

Description

When you know a certain area under a normal density curve, this function returns the x-axis values of the boundary of that area.

Usage

qnormGC(area,region="below",mean=0,sd=1,graph=FALSE)

Arguments

area

The known percentile

region

A character string. Default is "below". Other possible values are "between" (when known area is symmetric around the mean two numbers), "below", "above", and "outside" (when known area is outside a region symmetric around the mean)

mean

Mean of the distribution

sd

Standard deviation of the distribution

graph

Will produce graph of the area

Value

Numerical value of the percentile, and a vector when there are two bounds.

Author(s)

Homer White [e-mail]

Examples

#80th percentile of a normal distribution with mean=70 and sd=4:
qnormGC(0.80,region="below",mean=70,sd=4)

#Return value x so that P(X > x) = 0.10 (same as the 90th percentile)
qnormGC(0.10,region="above",mean=70,sd=4)

#This gives the multiplier for 95%-confidence intervals based on the z-statistic
qnormGC(0.95,region="between")

#This gives critical values for a two-sided z-test with alpha = 0.01:
qnormGC(0.01,region="outside")