City College, Spring 2019

Intro to Data Science

Week 10: Performance Evaluation and Ensemble Models

April 15, 2019

Today's Agenda
  1. Model Fit
  2. Anomaly Detection
  3. Ensemble Models
    • Random Forests
    • Gradient Boosting Machines
Part I

Evaluating Model Performance

Did we see this coming?

Democratic strategist David Axelrod predicted that poll performance “is going to prompt another round of soul-searching about whether and how you can poll accurately, because a lot of these races that were blowouts tonight or apparently blowouts tonight polled as tough races.”

Democratic pollster Andrew Baumann called the pre-election polls “quite accurate, particularly for a midterm that ended up being totally different than any previous midterm.”

Was this true?
538 Prediction Results
Actual Dem Actual Rep
Predicted Dem 226 2
Predicted Rep 9 197

Based on 538 deluxe model for 11/05/18.

What are the downsides of these measures?
Lift Charts

Rank all observations by the predicted probability class, and chart the cummulative share of actual true values captured by the first x observations, where x ranges from 1 to the total number of observations.

Demonstrates model's ability to outperform other (random) choices at positive prediction across decision thresholds.

For more details.
538 Lift
538 Lift
538 Lift
538 Lift
538 Lift
ROC Curves
ROC = Receiver Operating Characteristic

Plot the true positive rate against the false positive rate at every possible threshold from highest to lowest.

Demonstrates model's ability to outperform other (random) choices across decision thresholds while weighing false positives against false negatives.

For more details.
538 ROC
538 ROC
538 ROC
538 ROC
Can we reduce this to a single number?
AUC = Area Under the Curve

Total volume of area under the ROC curve.

Sci-kit Learn can calculate this for you.

A Digression

538 Prediction Results
Actual Dem Actual Rep Total
Predicted Dem 226 2 228
Predicted Rep 9 197 206
Total 235 199 434

Not everything went according to plan.
Anomaly Detection
The identification of rare items, events or observations which raise suspicions by differing significantly from the majority of the data.
Anomalies in NC-9
Part II

Ensemble Models

Classification Models
  1. Logistic Regression
  2. Decision Trees
  3. Random Forest
  4. Gradient Boosting Machine
  5. Naive Bayes
Review: Survival Rates Among Subgroups
Review: Survival Rates Among Subgroups
Ensemble Models

Ensemble methods use multiple learning algorithms to obtain better predictive performance than could be obtained from any of the constituent learning algorithms alone.

Random Forests

Random Forests construct a multitude of decision trees at training time and outputting the class that is the mode of the predicted classes among each tree.

For more details.
And a nice overview of everything.
Random Forests
Gradient Boosting

Gradient Boosting produces a prediction model in the form of an ensemble of weak prediction models and then generalizes them by allowing optimization of an arbitrary loss function.

For more details.
Gradient Boosting
Let's try.

Project Proposal: Due Monday, April 29 by 6:30pm

Due to (preferably as an attached pdf):

  1. A brief section addressing feedback from the project proposal.
  2. A 400 to 600 word summary of findings from the data exploration.
  3. At least two charts from exploratory data analysis.