The notion of prediction
Notation and convention
If the variables can be classified as an explanatory variable and a response, we use the letter X to denote the explanatory variable and Y to denote the response.
Always draw the response variable, Y, on the vertical axis of a scatterplot and X on the horizontal axis.
Predicting the response
The correlation coefficient describes the strength of a relationship, but does not help you to predict Y from X.
A curve or straight line that is drawn close to the crosses on a scatterplot (by eye or by any other method) is called a regression line and can be used to 'read off' the y-value corresponding to any x.