If sales go up when the advertising budget goes up, they correlate. The correlation coefficient is a measure of how closely the two data sets correlate. A correlation coefficient of 1 is a perfect correlation, .9 is a strong correlation, and .2 is a weak correlation. This value can also be negative, as when the incidence of a disease goes down when vaccinations go up. A correlation coefficient of -1 is a perfect negative correlation. Always remember, though, that correlation does not imply causation.