diff --git a/evaluation/confusion.go b/evaluation/confusion.go
index 3b00224..87fe2b1 100644
--- a/evaluation/confusion.go
+++ b/evaluation/confusion.go
@@ -11,19 +11,22 @@ type ConfusionMatrix map[string]map[string]int
 
 // GetConfusionMatrix builds a ConfusionMatrix from a set of reference (`ref')
 // and generate (`gen') Instances.
-func GetConfusionMatrix(ref *base.Instances, gen *base.Instances) map[string]map[string]int {
+func GetConfusionMatrix(ref base.FixedDataGrid, gen base.FixedDataGrid) map[string]map[string]int {
 
-	if ref.Rows != gen.Rows {
+	_, refRows := ref.Size()
+	_, genRows := gen.Size()
+
+	if refRows != genRows {
 		panic("Row counts should match")
 	}
 
 	ret := make(map[string]map[string]int)
 
-	for i := 0; i < ref.Rows; i++ {
-		referenceClass := ref.GetClass(i)
-		predictedClass := gen.GetClass(i)
+	for i := 0; i < int(refRows); i++ {
+		referenceClass := base.GetClass(ref, i)
+		predictedClass := base.GetClass(gen, i)
 		if _, ok := ret[referenceClass]; ok {
-			ret[referenceClass][predictedClass]++
+			ret[referenceClass][predictedClass] += 1
 		} else {
 			ret[referenceClass] = make(map[string]int)
 			ret[referenceClass][predictedClass] = 1