# How to favour a particular class during classification using XGBoost?

I am using a simple XGBoost model to classify 2 classes (0 and 1) in a binary context. In case of the original data, the 0 is the majority class and 1 the minority class. The thing which is happening is that in case of classification, most 0s are being classified correctly, with many going into 1s, but most 1s are being misclassified into 0s.

I am fairly new to this, and having looked at various documentations and questions on SE, am really confused as to how I can specify my XGBoost model to favour class 1 (to be precise, if most 0s are misclassified into 1s, that is not a problem, but I want that most 1s are correctly classified as 1s (to increase the true positives, if there are false positives that is something which isn't much of a problem). The segment of code I am presently using to train and test the XGBoost are as follows (afterwards I use the confusion matrix in which the true positives (1s) are highly misclassified into 0s).

from xgboost import XGBClassifier

# fit model on training data
model = XGBClassifier()
model.fit(X_train, labels) # where labels are either 1s or 0s

# make predictions for test data
y_pred = model.predict(X_test)
y_pred = y_pred > 0.70 # account for > 0.70 probability
y_pred = y_pred.astype(int)

print(y_pred)


I just want to know if there is a simple way to specify to the XGBoost model any parameter in my code, so that the true positive rate can be increased? I can compromise of false positives being high, but I want the number of 1s to be correctly classified as 1s, instead of most of them going into 0s. Any help in this regard is appreciated.

UPDATE:

I have now tried to use scale_pos_weight in the XGBoost, with its value set to 0.70 (a random figure), but it is still landing most samples to 0, instead of 1.

XGBoost has the scale_pos_weight parameter to help with this, depending on how you want to evaluate it (see tuning notes). It should be the ratio of negative count to positive count (or inverse based on how you indexed your classes).

An example in Python is here.

• Thanks a lot for your answer. It would be great if you could kindly give a small example of using ratio of negative count to positive count. Is it a fractional value in that sense? It would be helpful if you could give a one line example in using it within fit(). – JChat Mar 16 '19 at 9:41
• Also, unfortunately I couldn't find the use of scale_pos_weight in Python, but the documentation only mentions that in R. xgboost.readthedocs.io/en/latest/python/… this is the Python page but I am unable to understand how to use it in the current context please. – JChat Mar 16 '19 at 11:57
• The docs reference examples in Python, but I added a link to one in my answer. – wwwslinger Mar 16 '19 at 17:31
• Happy to accept your answer. However, I have now tried to use scale_pos_weight in the XGBoost, with its value set to 0.70 (a random figure), but it is still landing most samples to 0, instead of 1.. Any suggestions please? 0 is the majority class and 1 the minority one, and I want to maximise the predictions of 1s to be true, even if it leads to false positives. – JChat Mar 16 '19 at 17:40
• The value should be representative of the class distribution. See the example, try inverting the ratio, and try whole numbers. I think some examples I've seen had 9 when one class was 9 times more prevalent. – wwwslinger Mar 16 '19 at 17:51