At a very basic level, the relationship between a continuous response variable (Y) and a continuous explanatory variable(X) may be represented using a line of best-fit, where Y is predicted, at least to some extent, by X. If this relationship is linear, it may be appropriately represented mathematically using the straight line equation ‘Y=a+βx', as shown in Figure 1 (this line was computed using the least-squares procedure; see Ryan, 1997).