R-Square, also known as the Coefficient of determination is a commonly used statistic to evaluate model fit R-squareis 1 minus the ratio of residual variability. If X and Y are perfectly related then there is no residual variance and the ratio of variance would be 0.0, making R-square = 1. In most cases, the ratio and R-square will fall somewhere between these extremes, that is, between 0.0 and 1.0.