Working with the Assessment Window

About the Assessment Window

For a logistic regression, the Assessment window plots lift, ROC, and misclassification rates. Use the Assessment window to determine how well the model fits the data.

Lift is the ratio of the percent of captured responses within each percentile bin to the average percent of responses for the model. Similarly, cumulative lift is calculated by using all of the data up to and including the current percentile bin.

A receiver operating characteristic (ROC) chart displays the ability of a model to avoid false positive and false negative classifications. A false positive classification means that an observation has been identified as an event when it is actually a nonevent (also referred to as a Type I error). A false negative classification means that an observation has been identified as a nonevent when it is actually an event (also referred to as a Type II error).

The misclassification plot displays how many observations were correctly and incorrectly classified for each value of the response variable. When the response variable is not binary, the logistic regression model considers all levels that are not events as equal. A significant number of misclassifications could indicate that your model does not fit the data.

Use the Assessment Window

Lift

The default lift chart displays the cumulative lift of the model. To view the noncumulative lift, click Show Actions

on the vertical axis, and select Lift.

For comparison, the lift chart plots a best model based on complete knowledge of the input data.

ROC

The specificity of a model is the true negative rate. To derive the false positive rate, subtract the specificity from 1. The false positive rate, labeled 1 – Specificity, is the X axis of the ROC chart. The sensitivity of a model is the true positive rate. This is the Y axis of the ROC chart. Therefore, the ROC chart plots how the true positive rate changes as the false positive rate changes.

A good ROC chart has a very steep initial slope and levels off quickly. That is, for each misclassification of an observation, significantly more observations are correctly classified. For a perfect model, one with no false positives and no false negatives, the ROC chart would start at (0,0), continue vertically to (0,1), and then horizontally to (1,1). In this instance, the model would correctly classify every observation before a single misclassification could occur.

The ROC chart includes two lines to help you interpret the ROC chart. The first line is a baseline model that has a slope of 1. This line mimics a model that correctly classifies observations at the same rate it incorrectly classifies them. An ideal ROC chart maximizes the distance between the baseline model and the ROC chart. A model that classifies more observations incorrectly than correctly would fall below the baseline model. The second line is a vertical line at the false positive rate where the difference between the Kolmogorov-Smirnov values for the ROC chart and baseline models is maximized.