Correlations

When both of the variables under investigation are continuous, you ask the question 'is this correlated with that?'

For example, let's have another look at Figure 3.1, where we had a look at the relationship between age and body fat percentage. Figure 3.1 is called a scatter plot, and the best-fit line between the plot is called a line of regression.

This is an important graph because it gives you a good feel for your data and what the underlying story might be. When you see a scatter plot such as this, you should always ask the following questions:

• Is there a gradient to the regression line? — Is the gradient positive (slopes up from left to right)?, Or negative (slopes down from left to right)?
• Do the plot points fall close to or far from the regression line?
• Is the linear regression line a good representation of the data? — Would a curve or other line shape be a better fit?
• Are there clearly defined clusters of data?
• Are there...