chngpt {VDSPCalibration}

Piecewise Regression Estimation

Description

Estimate a piecewise linear regression equation

Usage

chngpt(x, y, start = quantile(x, probs = 0.1,

na.rm = "TRUE"),finish = quantile(x, probs = 0.9, na.rm = "TRUE"),

NbrSteps = 500)

Arguments

x

old VD measurements

y

reference (new) VD measurements

start

lower bound of the changing point

finish

upper bound of the changing point

NbrSteps

number of points used in grid search

Details

This function uses grid search method to fit a piecewise linear regression model with one changing point

Value

x

old VD levels

y

new VD levels

yfitted

calibrated VD levels based on the fitted piecewise linear regression

chngpt

the estimated chang point

coefficients

the estimated regression coefficients for the piecewise linear regression

Author(s)

Durazo-Arvizu, Ramon and Sempos, Chris

References

Tian L., Durazo-Arvizu R. A., Myers G., Brooks S., Sarafin K., and Sempos C. T. (2014), The estimation of calibration equations for variables with heteroscedastic measurement errors, Statist. Med., 33, pages 4420-4436

Examples

### Generate equally spaced TEST VALUES in the interval [20,200]
set.seed(123456789)
x= 20 + 180*1:100/100
x2= (x - 95)*(x>=95)

# Generate REFERENCE VALUES
y = -8 + 1.5*x - 0.85*x2 + 15*rnorm(100)

#Plot test values versus reference values along with fitted piecewise model
plot(x,y)
fit.chngpt = chngpt(x,y)
plot(fit.chngpt$x[order(fit.chngpt$yfitted)],
     fit.chngpt$y[order(fit.chngpt$yfitted)],
     xlim=c(0,200), ylim=c(0,200), xlab="25-Hydroxyvitamin D (nmol/mL), IDS",
     ylab="25-Hydroxyvitamin D (nmol/mL), LC/MS", bty="n", las=1)
lines(fit.chngpt$x[order(fit.chngpt$yfitted)],
      fit.chngpt$yfitted[order(fit.chngpt$yfitted)], lty=2,col=2, lwd=2)
abline(v=fit.chngpt$chngpt, lty=2,col=3, lwd=2)
arrows(fit.chngpt$chngpt+20 ,15, fit.chngpt$chngpt,-8, length=0.1, lwd=2, col=4)
legend(fit.chngpt$chngpt + 5,30, legend=round(fit.chngpt$chngpt, digits=1),
       bty="n", col=4)