Confusion matrix
To access the accuracy of an image classification, it is common practice to create a confusion matrix. In a confusion matrix, your classification results are compared to additional ground truth information.
To obtain a confusion matrix, you require a raster map with ground truth information (which has not been used to train the classifier); cross this map with your classification result, and display the cross table; finally, choose the Confusion matrix command from the View menu in the table window.
For more information, refer to How to calculate a confusion matrix.