Ask HN: efficiency, efficacy, effectiveness?!

The more I read about the subject the more confused I become.

I tried to identify the types of those "functions" and work out "definitions". Please correct/expand.

  Effectiveness 
  x -> Bool
  
  Efficacy
  x -> (0, 1)
  
  Efficiency
  x -> (0, 1)


  Effective = 100% efficacious

  Efficiency = essential / (essential + non-essential)