Likewise, a change in 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
   is not necessary to change y, because a change in y could be caused by something implicit in the error term (or by some other causative explanatory variable included in the model). The above way of testing for causality requires belief that there is no reverse causation, in which y would cause 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
  . This belief can be established in one of several ways.
Perhaps the most widely known cultural reference to pediculosis capitis, occurring in a noted poem by Robert Burns. Other animals
Lice infestation in general is known as pediculosis, and occurs in many mammalian and bird species. Lice infesting other host species are not the same organism as that which causes head lice infestations in humans, nor do the three louse species which infest humans infest any other host species. References
External links

CDC: Head lice