Which of the following is a number that is used to describe how two random variables are related

The correlation coefficient, often denoted by the lower case letter r, is a number used to describe the relationship between two variables. The correlation coefficient is a number between -1 and 1.

Let's call the variables A and B.

If as A goes up, B goes up then there is a positive correlation between A and B. Here the correlation coefficient would be positive. the closer it is to 1 the stronger the relationship between the two variables. One example of two variables that have a strong positive correlation is height and shoe size. As height increases so does shoe size.

If as A goes up B goes down and vice versa then there is a negative correlation. The closer to -1 the correlation coefficient is the stronger the correlation. Two variables that are negatively correlated are temperature and and swim suits sold. As the temperature increases the sale of swim suits decreases.

If there is no relationship between the variables then the correlation coefficient is 0.