R code to compute Cronbach's alpha, using the formula interface

cronbachs_alpha.R

cronbach_alpha = function(formula, data=NULL)
{
  terms = terms(formula, data=data)
  variables = attr(terms, "variables")
  var_list = eval(variables, envir=data)
  var_mat = t(laply(var_list, identity))
  var_mat = var_mat[complete.cases(var_mat),]
  response_idx = attr(terms, "response")
  response = var_mat[,response_idx]
  covariates = var_mat[,-response_idx]
  
  K = dim(covariates)[2]
  alpha = K/(K-1) * (1 - sum(diag(var(covariates)))/var(response))
  
  return(alpha)
}

# Example usage
cronbach_alpha(confidence_pa ~ PAtired + PAstress + PAdemands + PAdepress, data=long0)