uslopes: Estimates slopes at extremes

Description

This function estimates the slope of the relationship at the lower and upper extreme points of the independent variable

Usage

uslopes(lmObject, vars, .vcov = NULL, x.min = NULL, x.max = NULL)

Value

A list with class "uslopes" containing the following components:

interval: a vector with the interval for the independent varaible.
slope: a vector with the slope of the relationship at the extremes.
tval: a vector with the t-value of the relationship at the extremes.
pval: a vector with the p-value from a one-sided test a the extremes of the interval.
extreme: the estimated extreme point of the relationship.

Arguments

lmObject: The model to be tested
vars: A vector with the name of the linear and squared terms. Can also be provided as a formula
.vcov: The covariance matrix to use
x.min: Lower bound of interval. If NULL, the minimum observed in the data is used.
x.max: Upper bound of interval. If NULL, the maximum observed in the data is used.

Details

The function computes slopes of a quadratic relationship at the lower and upper bound defined by x.min and x.max. Standard errors of the estimated slopes, t-values, and p-values from a one-sided test of a flat relationship as well as the extreme point of the estimated relationship are also provided.

Examples

Run this code

x <- runif(100,min=-1,max=1)
xsq <- x^2
y <- x^2+rnorm(100)
mod <- lm(y~x+xsq)

uslopes(mod,c("x","xsq"))
uslopes(mod,~x+xsq,x.max=0.8)