A lurking variable is a variable that has an important effect on the relationship among the variables in a study but is not included among the variables studied
Common response:Both X and Y respond to changes in some unobserved variable.All three of our examples are examples of common response.
Ice cream sales and shark attacks both increase during summer.
Skirt lengths and stock prices are both controlled by the general attitude of the country,liberal or conservative.
The number of cavities and children's vocabulary are both related to a child's age.