Copyright 2013 © Limsoon Wong,

Why non-parametric methods

• The study variable generates data which are
  scores and so should be treated as a categorical
  variable with data measured on ordinal scale
  
  
  • E.g., scoring system for degree of skin reaction to
    a chemical agent:
    
    
    • 1: intense skin reaction
    
    
    • 2: less intense reaction
    
    
    • 3: No reaction
    
    
    
    
  
  
  
  


• For such type of data, the assumption required
  for parametric tests seem invalid => non-
  parametric methods should be used


• Aka distribution-free tests, because they make no
  assumption about the underlying distribution of
  the study variables