Notice that placentas are called endometrium more often than placenta. Note also that the number of placentas is just six, and that, by default, rpart requires 20 observations before splitting a node....

Notice that placentas are called endometrium more often than placenta. Note also that the number of placentas is just six, and that, by default, rpart requires 20 observations before splitting a node. Thus it is not possible with these parameters to have a node in which placentas are the majority. Rerun the above analysis but this time permit rpart to split any node by using the argument control = rpart.control(minsplit = 0). Does the accuracy increase? Look at the confusion matrix again.