A study revealed a strong, linear, positive association between the amount of shampoo used and the length of hair.

A study revealed a strong, linear, positive association between the amount of shampoo used and the length of hair. Another study revealed a strong, linear, positive association between amount of conditioner used and the length of hair. When the data were combined, the association became negative.

What is this an example of?

 A.Observational study

 B.Experimental study

 C.Simpson's Paradox

 D.Cause-and-effect relationship

E. Error by the statistician