varContrasts: Set Factor Contrasts

Description

Calculates contrast matrix for a specified contrast type. Options include DUMMY, POC, HELMERT, EFFECTS

Usage

varContrasts(TheFactor, Type = "DUMMY", RefLevel = length(levels(TheFactor)), 
       POCList = NULL, Labels = NULL)

Arguments

TheFactor

factor from dataframe

Type

type of contrast, Options include DUMMY (default), POC, HELMERT, or EFFECTS

RefLevel

Reference level for contrast. Only applies to DUMMY, HELMERT, and EFFECTS. For DUMMY: RefLevel is numeric index of control/reference category (i.e. coded 0 for all regressors). For HELMERT: RefLevel = 1 indicates reverse HELMERT (i.e., last vs. earlier, second to last vs. earlier, etc), RefLevel = 'Highest Level' indicates forward HELMERT (i.e., first vs. later, second vs. later, etc). For EFFECTS: RefLevel is numeric index of excluded level.

POCList

if Type = POC, a list of Contrasts is required in POCList; e.g., list(c(1,0,-1), c(-1,2,-1)). Best to provide as whole numbers. Function will re-scale to unit weighted contrasts.

Labels

if Type = POC, Labels can be provided. If NULL (Default), contrast labels are POC1, POC2, etc.

Value

Returns contrast matrix for indicated type of contrast.

Details

Use the contrast matrix with contrasts() to set contrast for a specific factor in dataframe.

Examples

Run this code

# NOT RUN {
  d = data.frame(f=factor(c('f1', 'f2', 'f3')))
  contrasts(d$f)
  
  ##set as DUMMY with last category as reference
  contrasts(d$f) = varContrasts(d$f, Type='DUMMY', RefLevel = 3) 
  
  ##set as POC with user defined labels
  contrasts(d$f) = varContrasts(d$f, Type='POC', POCList = list(c(2,-1,-1),c(0,1,-1)), 
                   Labels = c('f1_v_f2f3', 'f2_v_f3')) 
                   
  ##set as reverse HELMERT                 
  contrasts(d$f) = varContrasts(d$f, Type='HELMERT', RefLevel = 1) 
  
  ##set as EFFECTS, excluding f3 vs. grand mean contrast
  contrasts(d$f) = varContrasts(d$f, Type='EFFECTS', RefLevel = 3) 
# }