http://en.wikipedia.org/wiki/Confounding_variable confounding variables alternative explanations confounding variables neither http://en.wikipedia.org/wiki/Confounding_variable neither confounding variables http://en.wikipedia.org/wiki/Confounding_variable confounding variables confounding variables confounding variables confounding variable Confounding variables neither confounding variable Confounding variables http://en.wikipedia.org/wiki/Confounding_variable confounding variables confounding variable circumstances change confounding variables http://en.wikipedia.org/wiki/Confounding_variable confounding variable neither confounds alternative explanations confounding variables confounding variable confounding variable http://en.wikipedia.org/wiki/Confounding_variable this Wikipedia article confounding variable confounds neither Confounding variable – Wikipedia confounding variable confounding variables http://en.wikipedia.org/wiki/Confounding_variable