Question: I am trying code from this page. I ran up to the part LR (tf-idf) and got the similar results After that I decided to try GridSearchCV. My questions below: 1) #lets try gridsearchcv #https://www.kaggle.com/enespolat/grid-search-with-logistic-regression from sklearn.model_selection import GridSearchCV grid={"C":np.logspace(-3,3,7), "penalty":["l2"]}# l1 lasso l2 ridge logreg=LogisticRegression(solver = 'liblinear') logreg_cv=GridSearchCV(logreg,grid,cv=3,scoring='f1') logreg_cv.fit(X_train_vectors_tfidf, y_train) print("tuned hpyerparameters :(best parameters) ",logreg_cv.best_params_) print("best score :",logreg_cv.best_score_) #tuned hpyerparameters :(best parameters) {'C': 10.0, 'penalty': 'l2'} #best score : 0.7390325593588823  Then I calculated f1 score manually. why it is not matching? logreg_cv.predict_proba(X_train_vectors_tfidf)[:,1] final_prediction=np.where(logreg_cv.predict_proba(X_train_vectors_tfidf)[:,1]>=0.5,1,0) #https://www.statology.org/f1-score-in-python/ from sklearn.metrics import f1_score #calculate F1 score f1_score(y_train, final_prediction) 0.9839388145315489   If I try scoring='precision' why does it give below error? I am not clear mainly because I have relatively balanced dataset (55-45%) and f1 which requires precision is getting calculated without any problems  #lets try gridsearchcv #https://www.kaggle.com/enespolat/grid-search-with-logistic-regression from sklearn.model_selection import GridSearchCV grid={"C":np.logspace(-3,3,7), "penalty":["l2"]}# l1 lasso l2 ridge logreg=LogisticRegression(solver = 'liblinear') logreg_cv=GridSearchCV(logreg,grid,cv=3,scoring='precision') logreg_cv.fit(X_train_vectors_tfidf, y_train) print("tuned hpyerparameters :(best parameters) ",logreg_cv.best_params_) print("best score :",logreg_cv.best_score_) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) /usr/local/lib/python3.7/dist-packages/sklearn/metrics/_classification.py:1308: UndefinedMetricWarning: Precision is ill-defined and being set to 0.0 due to no predicted samples. Use `zero_division` parameter to control this behavior. _warn_prf(average, modifier, msg_start, len(result)) tuned hpyerparameters :(best parameters) {'C': 0.1, 'penalty': 'l2'} best score : 0.9474200393672962   is there any easier way to get predictions on the train data back? we already have the logreg_cv object. I used below method to get the predictions back. Is there a better way to do the same?  logreg_cv.predict_proba(X_train_vectors_tfidf)[:,1] ############################ ############update 1  Please answer question 1 from above. In the comment for the question it says The best score in GridSearchCV is calculated by taking the average score from cross validation for the best estimators. That is, it is calculated from data that is held out during fitting. From what I can tell, you are calculating predicted values from the training data and calculating an F1 score on that. Since the model was trained on that data, that is why the F1 score is so much larger compared to the results in the grid search  is that the reason I get below results #tuned hpyerparameters :(best parameters) {'C': 10.0, 'penalty': 'l2'} #best score : 0.7390325593588823 but when i do manually i get f1_score(y_train, final_prediction) 0.9839388145315489 2) I tried to tune using f1_micro as suggested in the answer below. No error message. I am still not clear why f1_micro is not failing when precision  fails from sklearn.model_selection import GridSearchCV grid={"C":np.logspace(-3,3,7), "penalty":["l2"], "solver":['liblinear','newton-cg'], 'class_weight':[{ 0:0.95, 1:0.05 }, { 0:0.55, 1:0.45 }, { 0:0.45, 1:0.55 },{ 0:0.05, 1:0.95 }]}# l1 lasso l2 ridge #logreg=LogisticRegression(solver = 'liblinear') logreg=LogisticRegression() logreg_cv=GridSearchCV(logreg,grid,cv=3,scoring='f1_micro') logreg_cv.fit(X_train_vectors_tfidf, y_train) tuned hpyerparameters :(best parameters) {'C': 10.0, 'class_weight': {0: 0.45, 1: 0.55}, 'penalty': 'l2', 'solver': 'newton-cg'} best score : 0.7894909688013136 

Answer: You end up with the error with precision because some of your penalization is too strong for this model, if you check the results, you get 0 for f1 score when C = 0.001 and C = 0.01 res = pd.DataFrame(logreg_cv.cv_results_) res.iloc[:,res.columns.str.contains("split[0-9]_test_score|params",regex=True)] params split0_test_score split1_test_score split2_test_score 0 {'C': 0.001, 'penalty': 'l2'} 0.000000 0.000000 0.000000 1 {'C': 0.01, 'penalty': 'l2'} 0.000000 0.000000 0.000000 2 {'C': 0.1, 'penalty': 'l2'} 0.973568 0.952607 0.952174 3 {'C': 1.0, 'penalty': 'l2'} 0.863934 0.851064 0.836449 4 {'C': 10.0, 'penalty': 'l2'} 0.811634 0.769547 0.787838 5 {'C': 100.0, 'penalty': 'l2'} 0.789826 0.762162 0.773438 6 {'C': 1000.0, 'penalty': 'l2'} 0.781003 0.750000 0.763871  You can check this: lr = LogisticRegression(C=0.01).fit(X_train_vectors_tfidf,y_train) np.unique(lr.predict(X_train_vectors_tfidf)) array([0])  And that the probabilities predicted drift towards the intercept: # expected probability np.exp(lr.intercept_)/(1+np.exp(lr.intercept_)) array([0.41764462]) lr.predict_proba(X_train_vectors_tfidf) array([[0.58732636, 0.41267364], [0.57074279, 0.42925721], [0.57219143, 0.42780857], ..., [0.57215605, 0.42784395], [0.56988186, 0.43011814], [0.58966184, 0.41033816]])  For the question on "get predictions on the train data back", i think that's the only way. The model is refitted on the whole training set using the best parameters, but the predictions or predicted probabilities are not stored. If you are looking for the values obtained during train / test, you can check cross_val_predict