Multi-class classification¶
Classification is about predicting an outcome from a fixed list of classes. The prediction is a probability distribution that assigns a probability to each possible outcome.
A labeled classification sample is made up of a bunch of features and a class. The class is a usually a string or a number in the case of multiclass classification. We'll use the image segments dataset as an example.
from river import datasets
dataset = datasets.ImageSegments()
dataset
Image segments classification.
This dataset contains features that describe image segments into 7 classes: brickface, sky,
foliage, cement, window, path, and grass.
Name ImageSegments
Task Multi-class classification
Samples 2,310
Features 18
Sparse False
Path /Users/max.halford/projects/river/river/datasets/segment.csv.zip
This dataset is a streaming dataset which can be looped over.
for x, y in dataset:
pass
Let's take a look at the first sample.
x, y = next(iter(dataset))
x
{'region-centroid-col': 218,
'region-centroid-row': 178,
'short-line-density-5': 0.11111111,
'short-line-density-2': 0.0,
'vedge-mean': 0.8333326999999999,
'vegde-sd': 0.54772234,
'hedge-mean': 1.1111094,
'hedge-sd': 0.5443307,
'intensity-mean': 59.629630000000006,
'rawred-mean': 52.44444300000001,
'rawblue-mean': 75.22222,
'rawgreen-mean': 51.22222,
'exred-mean': -21.555555,
'exblue-mean': 46.77778,
'exgreen-mean': -25.222220999999998,
'value-mean': 75.22222,
'saturation-mean': 0.31899637,
'hue-mean': -2.0405545}
y
'path'
A multiclass classifier's goal is to learn how to predict a class y from a bunch of features x. We'll attempt to do this with a decision tree.
from river import tree
model = tree.HoeffdingTreeClassifier()
model.predict_proba_one(x)
{}
The reason why the output dictionary is empty is because the model hasn't seen any data yet. It isn't aware of the dataset whatsoever. If this were a binary classifier, then it would output a probability of 50% for True and False because the classes are implicit. But in this case we're doing multiclass classification.
Likewise, the predict_one method initially returns None because the model hasn't seen any labeled data yet.
print(model.predict_one(x))
None
If we update the model and try again, then we see that a probability of 100% is assigned to the 'path' class because that's the only one the model is aware of.
model.learn_one(x, y)
model.predict_proba_one(x)
{'path': 1.0}
This is a strength of online classifiers: they're able to deal with new classes appearing in the data stream.
Typically, an online model makes a prediction, and then learns once the ground truth reveals itself. The prediction and the ground truth can be compared to measure the model's correctness. If you have a dataset available, you can loop over it, make a prediction, update the model, and compare the model's output with the ground truth. This is called progressive validation.
from river import metrics
model = tree.HoeffdingTreeClassifier()
metric = metrics.ClassificationReport()
for x, y in dataset:
y_pred = model.predict_one(x)
model.learn_one(x, y)
if y_pred is not None:
metric.update(y, y_pred)
metric
Precision Recall F1 Support
brickface 77.13% 84.85% 80.81% 33
cement 78.92% 83.94% 81.35% 33
foliage 67.00% 20.30% 31.16% 33
grass 100.00% 96.97% 98.46% 33
path 90.39% 91.49% 90.94% 329
sky 99.08% 98.18% 98.63% 33
window 43.50% 67.88% 53.02% 33
Macro 79.43% 77.66% 76.34%
Micro 77.65% 77.65% 77.65%
Weighted 79.43% 77.65% 76.33%
77.65% accuracy
This is a common way to evaluate an online model. In fact, there is a dedicated evaluate.progressive_val_score function that does this for you.
from river import evaluate
model = tree.HoeffdingTreeClassifier()
metric = metrics.ClassificationReport()
evaluate.progressive_val_score(dataset, model, metric)
Precision Recall F1 Support
brickface 77.13% 84.85% 80.81% 33
cement 78.92% 83.94% 81.35% 33
foliage 67.00% 20.30% 31.16% 33
grass 100.00% 96.97% 98.46% 33
path 90.39% 91.49% 90.94% 329
sky 99.08% 98.18% 98.63% 33
window 43.50% 67.88% 53.02% 33
Macro 79.43% 77.66% 76.34%
Micro 77.65% 77.65% 77.65%
Weighted 79.43% 77.65% 76.33%
77.65% accuracy
That concludes the getting started introduction to multiclass classification! You can now move on to the next steps.