Inference, estimation, and decision-making from data
So far each variable stood alone. The real questions usually involve two variables at once: does study time relate to grades? does model size relate to accuracy? The first tool is a scatter plot (one dot per observation, x against y), which lets your eye spot a trend instantly.
To put a number on a linear trend, use the Pearson correlation coefficient r. It runs from −1 to +1: +1 is a perfect upward line, −1 a perfect downward line, 0 no linear relationship at all.
In the figure, the more tightly the points hug the fitted line, the closer |r| is to 1. Spread them out and r drifts toward 0.