plotDerivative {cNORM}

Plot first order derivative of regression model

Description

Plots the scores obtained via the first order derivative of the regression model in dependence of the norm score. The results indicate the progression of the norm scores within each age group. The regression based modeling approach relies on the assumption of a linear progression of the norm scores. Negative scores in the first order derivative indicate a violation of this assumption. Scores near zero are typical for bottom and ceiling effects in the raw data. The regression models usually converge within the range of the original values. In case of vertical and horizontal extrapolation, with increasing distance to the original data, the risk of assumption violation increases as well. ATTENTION: plotDerivative is currently still incompatible with reversed raw score scales ('descent' option)

Usage

plotDerivative(
  model,
  minAge = NULL,
  maxAge = NULL,
  minNorm = NULL,
  maxNorm = NULL,
  stepAge = 0.2,
  stepNorm = 1,
  order = 1
)

Arguments

model

The model from the bestModel function or a cnorm object

minAge

Age to start with checking

maxAge

Upper end of the age check

minNorm

Lower end of the norm score range, in case of T scores, 25 might be good

maxNorm

Upper end of the norm score range, in case of T scores, 25 might be good

stepAge

Stepping parameter for the age check, usually 1 or 0.1; lower values indicate higher precision / closer checks

stepNorm

Stepping parameter for norm scores

order

Degree of the derivative (default = 1)

See Also

checkConsistency, bestModel, derive

Other plot: plot.cnorm(), plotDensity(), plotNormCurves(), plotNorm(), plotPercentileSeries(), plotPercentiles(), plotRaw(), plotSubset()

Examples

# Load example data set, compute model and plot results
result <- cnorm(raw = elfe$raw, group = elfe$group)
plotDerivative(result, minAge=2, maxAge=5, step=.2, minNorm=25, maxNorm=75, stepNorm=1)