Let's unpack some of those [terms](https://scikit-learn.org/stable/modules/generated/sklearn.metrics.classification_report.html?highlight=classification_report#sklearn.metrics.classification_report) with a confusion matrix to help us measure the performance of our mdoel:
## Better comprehension via a confusion matrix
While you can get a scoreboard report [terms](https://scikit-learn.org/stable/modules/generated/sklearn.metrics.classification_report.html?highlight=classification_report#sklearn.metrics.classification_report) by printing out the items above, you might be able to understand your model more easily by using a [confusion matrix]() to help us understand how the model is performing.
> 🎓 A '[confusion matrix](https://en.wikipedia.org/wiki/Confusion_matrix)' (or 'error matrix') is a table that expresses your model's true vs. false positives and negatives, thus gauging the accuracy of predictions.
```python
from sklearn.metrics import confusion_matrix
confusion_matrix(y_test, predictions)
```
Take a look at our confusion matrix:
Take a look at your model's confusion matrix:
```
array([[162, 4],
[ 33, 0]])
```
Let's understand what these numbers mean with an example. Let's say out model can classify between two categories, category 0 and category 1. If your model predicts something as category 0 and it belongs to category 0 in reality we call it a true positive, shown by the top left number. If your model predicts something as category 1 and it belongs to category 0 in reality we call it a false positive, shown by the top right number. If your model predicts something as category 0 and it belongs to category 1 in reality we call it a false negative, shown by the bottom left number. If your model predicts something as category 0 and it belongs to category 0 in reality we call it a true negative, shown by the top left number.
What's going on here? Let's say our model is asked to classify items between two binary categories, category 'pumpkin' and category 'not-a-pumpkin'.
- If your model predicts something as a pumpkin and it belongs to category 'pumpkin' in reality we call it a true positive, shown by the top left number.
- If your model predicts something as not a pumpkin and it belongs to category 'pumpkin' in reality we call it a false positive, shown by the top right number.
- If your model predicts something as a pumpkin and it belongs to category 'not-a-pumpkin' in reality we call it a false negative, shown by the bottom left number.
- If your model predicts something as not a pumpkin and it belongs to category 'not-a-pumpkin' in reality we call it a true negative, shown by the bottom right number.
As you might have guessed we like to have a larger number of true positives and true negatives and a lower number of false negatives and false positives, which implies that the model performs better.
> Infographic by [Jen Looper](https://twitter.com/jenlooper)
Let's now understand more about the terms we saw earlier with the help of confusion matrix:
As you might have guessed it's preferable to have a larger number of true positives and true negatives and a lower number of false positives and false negatives, which implies that the model performs better.
✅ Q: According to the confusion matrix, how did the model do? A: Not too bad; there are a good number of true positives but also several false negatives.
Let's revisit the terms we saw earlier with the help of the confusion matrix's mapping of TP/TN and FP/FN:
🎓 Precision: TP/(TP + FN) The fraction of relevant instances among the retrieved instances (e.g. which labels were well-labeled)
🎓 Recall: TP/(TP + FP) The fraction of relevant instances that were retrieved, whether well-labeled or not
🎓 f1-score: (2 * precison * recall)/(precision + recall) A weighted average of the precision and recall, with best being 1 and worst being 0
🎓 f1-score: (2 * precision * recall)/(precision + recall) A weighted average of the precision and recall, with best being 1 and worst being 0
🎓 Support: The number of occurrences of each label retrieved
@ -205,8 +218,7 @@ Let's now understand more about the terms we saw earlier with the help of confus
🎓 Weighted Avg: The calculation of the mean metrics for each label, taking label imbalance into account by weighting them by their support (the number of true instances for each label).
> Can you think which metric you should use if you want your model to reduce the number of false negatives?
✅ Can you think which metric you should watch if you want your model to reduce the number of false negatives?
## Visualize the ROC Curve of this Model
This is not a bad model; its accuracy is in the 80% range so ideally you could use it to predict the color of a pumpkin given a set of variables.
Jen Looper
4 years ago
