the call plothousing('fair'produces the plot cum hoc ergo propter hoc it has been shown that college students who regularly attend class have higher average grades than students who attend class only sporadically those of us who teach these classes would like to believe that this is because the students learn something from the lectures of courseit is at least equally likely that those students get better grades because students who are more likely to attend classes are also more likely to study hard when two things are correlated, there is temptation to assume that one has caused the other consider the incidence of flu in north america the number of cases rises and falls in predictable pattern there are almost no cases in the summerthe number of cases starts to rise in the early falland then starts dropping as summer approaches now consider the number of children attending school there are very few children in school in the summerenrollment starts to rise in the early falland then drops as summer approaches the correlation between the opening of schools and the rise in the incidence of flu is inarguable this has led many to conclude that that going to school is an important causative factor in the spread of flu that might be truebut one cannot conclude it based simply on the correlation correlation does not imply causationafter allthe correlation could be used just as easily to justify the belief that flu outbreaks cause schools to be in session or perhaps there is no causal relationship in either directionand there is some lurking variable that statisticianslike attorneys and physicianssometimes use latin for no obvious reason other than to seem erudite this phrase means"with thistherefore because of this correlation is measure of the degree to which two variables move in the same direction if when goes up goes upthe variables are positively correlated if they move in opposite directions they are negatively correlated if there is no relationshipthe correlation is people' heights are positively correlated with the heights of their parents the correlation between hours spent playing video games and grade point average is negative