Assignment 1 classification algorithms


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Assignment 1
This assignment involves trying out a variety of classification algorithms and at least one clustering algorithm. It
requires use of python, numpy, matplotlib, and sklearn, so it also serves as an introduction to all those
tools. You can run python any way you like – one option is to do the development in a jupyter notebook but
there are others.
You should produce and submit EITHER
a text report as a .pdf file, including figures and tables as needed, PLUS code as a separate file, OR
a single jupyter notebook, as a .ipynb file
Whichever one you choose, submit it using the usual submission system. If you choose the notebook, make sure it
has already run (the marker won’t want to run it and wait to see what it produces!), and remember to use
markdown cells to write text, as per Tutorial 1, not comments in code cells.
And if your code ends up very verbose, consider the separate text report option above, to make it more readable.
Part one: classifiers
This part is inspired by plot_classifier_comparison which uses sklearn to build a 3-by-10 table of Classifiers
versus Data. In that case, the end result of each test was a single 2-dimensional plot, but in this assignment, the
goal for each such combination is instead a box plot. The x-axis gives options for a parameter of the model, while
the y-axis indicates the spread for a simple performance measure of the algorithm, namely the fraction of a test set
that it gets correct.
Set up
Start by downloading the following datasets (e.g. as csv files)
you will also be testing out models using “fake” data you make yourself, for which
you will need to import make_classification from sklearn.datasets . The
following code makes appropriate “fake data”:
Use that to make your fake data. Notice how it assigns labels and then moves the X around randomly, in order to
add “noise” to the data.
Preprocess the X data first, using X = StandardScaler().fit_transform(X) .
You will be trying out the following models, in sklearn :
KNeighborsClassifier (k nearest neighbours)
GaussianNB (the Gaussian form of Naive Bayes)
DecisionTreeClassifier (a decision tree (DT))
LogisticRegression (essentially, a perceptron)
GradientBoostingClassifier (Gradient Boosted DTs)
RandomForestClassifier (random forest, ensemble of DTs)
MLPClassifier (neural net)
You are to try out these classifiers on the above datasets, using their default settings unless otherwise specified.
The assignment is to investigate one complexity control parameter for each classifier, by setting it to a range of
plausible values and seeing how well it does on “held out” data. To do this you will need train_test_split
from sklearn .
If we were to do this (split the dataset into train and test) only once, we would only get a single estimate of
performance. To get better estimates, simply repeat at least 250 times with different random splits. For simplicity,
use a 50:50 train:test split in all cases. For each setting of the control parameter, we then have a distribution over
250 or so values to convey. A nice way to do this is to produce what is called a box plot (or box and whiskers plot):
in sklearn it is boxplot.
Produce the plots
Although it won’t fit into one summary figure, you’re essentially building a big table like the
plot_classifier_comparison one. However you should structure it as separate main plots (one per model/algorithm),
each consisting of several subplots (the different datasets).
In each case, the aim is adjust a parameter that has some effect on the “complexity” or modelling power of the
Classifier. In some cases there are multiple options here, so for consistency across the class the table below gives
the one you should try, and a sensible range of values for it.
# making a fake data set here.
X, y = make_classification(n_features=20, n_redundant=0, n_informative=5,
X += 4.0 * numpy.random.uniform(size=X.shape)
myfakedataset = (X,y)
Classifier controller values
KNeighborsClassifier “n_neighbors” 1 to 5
GaussianNB “var_smoothing” [1e-9,1e-5,1e-1])
LogisticRegression “C” [.1,.5,1.0,2.0, 5.0])
DecisionTreeClassifier “max_depth” 1 to 10
GradientBoostingClassifier “max_depth” 1 to 10
RandomForestClassifier “max_depth” 1 to 10
MLPClassifier “alpha” [1e-5, 1e-3,0.1, 10.0])
note: the GaussianNB case is a bit weird: var_smoothing is not a very obvious control parameter. But for
completeness, use it anyway.
Additional commentary following the plots
Include two summary tables, with rows being models, and columns being datasets:
Table I is to contain the best average value (of validation error).
Table II is to contain the associated (ie. best) value for the control parameter.
Write a paragraph summarising the overall results, as captured in these two tables.
If you notice something unexpected, point it out, explaining why you think it is worth mentioning.
Part one is worth 70% of the total marks for this assignment. The breakdown is 50 for the main plots, 10 for the
tables, and 10 for the discussion of them.
Parttwo: clustering, for semi-supervised learning
Worth 30% of the total marks for this assignment.
The real world often throws up situations in whic we have a mountain of unlabelled data but only relatively few
examples with their labels. Ultimately this is because acquiring accurate labels is expensive, since it often requires
human expertise, whereas acquiring unlabelled vectors is often automatic and cheap.
This part asks you to investigate whether it is possible to use features derived from unsupervised learning to
enhance the predictions made by a supervised learner (in this case, a Classifier).
The idea of semi-supervised learning is that, by using the results of unsupervised learning, we might be able to
improve those of supervised learning. Suppose we perform clustering on the total data available (including the
“test” – this is okay provided we don’t use the labels when clustering). We could include this cluster label (e.g. as
an integer) in an additional column of X. Might a classifier trained on this augmented X actually do better (on the
test data) than it would if trained only on the original X?
We might expect that semi-supervised learning should only improve performance in cases where there is some
legitimate causal connection between the process that generates the X patterns and the process that assigns the Y
classes. Your task is to see whether this is so, for a particular dataset.
To do the test, act as if almost all of the data is missing its label: you will use the iris data and split it so that 95
percent of it goes into a test set, and thus only 5 percent is available as a training set.
use KMeans(n_clusters=3) (ie. k-means with k=3) as the cluster method, run on the whole dataset, and add
the cluster label as a new column in X. Then,
for at least a hundred repeats:
randomly split the iris dataset (as above)
train GaussianNB() Classifiers on the training data (nb. you are welcome to optionally try other clustering
methods as well, but begin with this one)
without the cluster column, and
with the cluster column
find and store the test-set scores of (i) and (ii)
plot a scatterplot of the (i)-scores on x-axis against the (ii)-scores on the y-axis. So if the cluster label tends to help,
there will be more points above the diagonal than below it.
Finally, repeat the analysis for the `banknotes` data in place of `iris`, and write a paragraph explaining whether they
are the same or different, and why .
NOTE: additionally, for Students doing AIML421 only: (extra 20%)
see whether you can demonstrate the same effect, but via a dimensionality reduction algorithm instead of
clustering . To do so, it is likely you will need to use a dataset with more dimensions than ‘iris’.
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