Hence, a mathematically perfect correlation does not mean causation: it simply means that two variables are very highly correlated. This may even be the result of a spurious or illusory correlation due to the influence of a third variable, called a “lurking variable”. While students’ university qualifications are highly correlated with their income later in life (the higher the grades, the higher the salary), this correlation could be due to a third (lurking or hidden) variable, such as the tendency to work hard.
