TDC2017 | São Paulo - Trilha Java EE How we figured out we had a SRE team at - Feature Engineering

Lead Data Scientist DSc. student
TDC 2017

Data
Models
Features
Useful attributes for your modeling task

"Feature engineering is the process of
transforming raw data into features that better
represent the underlying problem to the
predictive models, resulting in improved
model accuracy on unseen data."
Jason Brownlee

? ML Ready
dataset
Features Model Task
Raw data

Here are some Feature Engineering techniques
for your Data Science toolbox...

Dataset
● Sample of users page views
and clicks during
14 days on June, 2016
● 2 Billion page views
● 17 million click records
● 700 Million unique users
● 560 sites
Can you predict which recommended content each user will click?

More details of my solution in this post series

Numerical
Spatial
Temporal
Categorical
Target

Homogenize missing values and different types of in the same feature, fix input errors, types, etc.
Original data
Cleaned data

Fields (Features)
Instances
Tabular data (rows and columns)
● Usually denormalized in a single file/dataset
● Each row contains information about one instance
● Each column is a feature that describes a property of the instance

Necessary when the entity to model is an aggregation from the provided data.
Original data (list of playbacks)
Aggregated data (list of users)

Necessary when the entity to model is an aggregation from the provided data.
Aggregated data with pivoted columns
Original data
# playbacks by device Play duration by device

● Usually easy to ingest by mathematical models.
● Can be prices, measurements, counts, ...
● Easier to impute missing data
● Distribution and scale matters to many models

● Datasets contain missing values, often encoded as blanks, NaNs or other
placeholders
● Ignoring rows and/or columns with missing values is possible, but at the price of
loosing data which might be valuable
● Better strategy is to infer them from the known part of data
● Strategies
○ Mean: Basic approach
○ Median: More robust to outliers
○ Mode: Most frequent value
○ Using a model: Can expose algorithmic bias

● Transform discrete or continuous numeric features in binary features
Example: Number of user views of the same document
>>> from sklearn import preprocessing
>>> X = [[ 1., -1., 2.],
... [ 2., 0., 0.],
... [ 0., 1., -1.]]
>>> binarizer =
preprocessing.Binarizer(threshold=1.0)
>>> binarizer.transform(X)
array([[ 1., 0., 1.],
[ 1., 0., 0.],
[ 0., 1., 0.]])
Binarization with scikit-learn

● Split numerical values into bins and encode with a bin ID
● Can be set arbitrarily or based on distribution
● Fixed-width binning
Does fixed-width binning make sense for this long-tailed distribution?
Most users (458,234,809 ~ 5*10^8) had only 1 pageview during the period.

● Adaptative or Quantile binning
Divides data into equal portions (eg. by median, quartiles, deciles)
>>> deciles = dataframe['review_count'].quantile([.1, .2, .3, .4, .5, .6, .7,
.8, .9])
>>> deciles
0.1 3.0
0.2 4.0
0.3 5.0
0.4 6.0
0.5 8.0
0.6 12.0
0.7 17.0
0.8 28.0
0.9 58.0
Quantile binning with Pandas

Compresses the range of large numbers and expand the range of small numbers.
Eg. The larger x is, the slower log(x) increments.

Histogram of # views by user Histogram of # views by user
smoothed by log(1+x)
Smoothing long-tailed data with log

● Models that are smooth functions of input features are sensitive to the scale
of the input (eg. Linear Regression)
● Scale numerical variables into a certain range, dividing values by a
normalization constant (no changes in single-feature distribution)
● Popular techniques
○ MinMax Scaling
○ Standard (Z) Scaling

● Squeezes (or stretches) all values within the range of [0, 1] to add robustness to
very small standard deviations and preserving zeros for sparse data.
>>> X_train = np.array([[ 1., -1., 2.],
... [ 2., 0., 0.],
... [ 0., 1., -1.]])
...
>>> min_max_scaler =
preprocessing.MinMaxScaler()
>>> X_train_minmax =
min_max_scaler.fit_transform(X_train)
>>> X_train_minmax
array([[ 0.5 , 0. , 1. ],
[ 1. , 0.5 , 0.33333333],
[ 0. , 1. , 0. ]])
Min-max scaling with scikit-learn

After Standardization, a feature has mean of 0 and variance of 1 (assumption of
many learning algorithms)
>>> import numpy as np
>>> X = np.array([[ 1., -1., 2.],
... [ 2., 0., 0.],
... [ 0., 1., -1.]])
>>> X_scaled = preprocessing.scale(X)
>>> X_scaled
array([[ 0. ..., -1.22..., 1.33...],
[ 1.22..., 0. ..., -0.26...],
[-1.22..., 1.22..., -1.06...]])
>> X_scaled.mean(axis=0)
array([ 0., 0., 0.])
>>> X_scaled.std(axis=0)
array([ 1., 1., 1.])
Standardization with scikit-learn

● Simple linear models use a linear combination of the individual input
features, x1
, x2
, ... xn
to predict the outcome y.
y = w1
x1
+ w2
x2
+ ... + wn
xn
● An easy way to increase the complexity of the linear model is to create
feature combinations (nonlinear features).
Area (m2)Example (House Pricing Prediction)
Degree 2 interaction features for vector x = (x1,
x2
)
y = w1
x1
+ w2
x2
+ w3
x1
x2
+ w4
x1
2
+ w4
x2
2
# Rooms
Price

>>> import numpy as np
>>> from sklearn.preprocessing import PolynomialFeatures
>>> X = np.arange(6).reshape(3, 2)
>>> X
array([[0, 1],
[2, 3],
[4, 5]])
>>> poly = poly = PolynomialFeatures(degree=2, interaction_only=False,
include_bias=True)
>>> poly.fit_transform(X)
array([[ 1., 0., 1., 0., 0., 1.],
[ 1., 2., 3., 4., 6., 9.],
[ 1., 4., 5., 16., 20., 25.]])
Polynomial features with scikit-learn

● Nearly always need some treatment to be suitable for models
● Examples:
Platform: [“desktop”, “tablet”, “mobile”]
Document_ID or User_ID: [121545, 64845, 121545]
● High cardinality can create very sparse data
● Difficult to impute missing

● Transform a categorical feature with m possible values into m binary features.
● If the variable cannot be multiple categories at once, then only one bit in the
group can be on.
● Sparse format is memory-friendly
● Example: “platform=tablet” can be sparsely encoded as “2:1”

● Common in applications like targeted advertising and fraud detection
● Example:
Some large categorical features from Outbrain Click Prediction competition

● Hashes categorical values into vectors with fixed-length.
● Lower sparsity and higher compression compared to OHE
● Deals with new and rare categorical values (eg: new user-agents)
● May introduce collisions
100 hashed columns

● Instead of using the actual categorical value, use a global statistic of this
category on historical data.
● Useful for both linear and non-linear algorithms
● May give collisions (same encoding for different categories)
● Be careful about leakage

or or
Counts Click-Through Rate
P(click | ad) = ad_clicks / ad_views

Factors to consider:
● Multiple time zones in some countries
● Daylight Saving Time (DST)
○ Start and end DST dates

● Apply binning on time data to make it categorial and more general.
● Binning a time in hours or periods of day, like below.
● Extraction: weekday/weekend, weeks, months, quarters, years...
Hour range Bin ID Bin Description
[5, 8) 1 Early Morning
[8, 11) 2 Morning
[11, 14) 3 Midday
[14, 19) 4 Afternoon
[19, 22) 5 Evening
[22-24) and (00-05] 6 Night

● Instead of encoding: total spend, encode things like:
Spend in last week, spend in last month, spend in last
year.
● Gives a trend to the algorithm: two customers with equal
spend, can have wildly different behavior — one
customer may be starting to spend more, while the other
is starting to decline spending.

● Spatial variables encode a location in space, like:
○ GPS-coordinates (lat. / long.) - sometimes require projection to a different
coordinate system
○ Street Addresses - require geocoding
○ ZipCodes, Cities, States, Countries - usually enriched with the centroid
coordinate of the polygon (from external GIS data)
● Derived features
○ Distance between a user location and searched hotels (Expedia competition)
○ Impossible travel speed (fraud detection)

Reduces model complexity and training time
● Filtering - Eg. Correlation our Mutual Information between
each feature and the response variable
● Wrapper methods - Expensive, trying to optimize the best
subset of features (eg. Stepwise Regression)
● Embedded methods - Feature selection as part of model
training process (eg. Feature Importances of Decision Trees or
Trees Ensembles)

Outbrain Click Prediction - Leaderboard score of my approaches

Towards Automated Feature
Engineering
Deep Learning....

Questions?
Gabriel Moreira
Lead Data Scientist
@gspmoreira
Versão extendida:
bit.ly/feature_eng_tdc

TDC2017 | São Paulo - Trilha Java EE How we figured out we had a SRE team at - Feature Engineering

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