You can plot points and plot the resulting best-fit line determined in the previous section but the question remains whether the line is any good. In particular, the real use of the line often is to subsequently predict y-values for a given x-value. However, it is very likely that the best-fit line does not even pass through any of the provided data points. So, how can something that misses every marker still be considered a good fit. To quantify this, we first need to discuss a way to measure how well two variables vary with each other.