The zen of predictive modelling

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The Zen of Predictive Modelling

Eugene [email protected]+61414573322@cargomoose

mailto:[email protected]

What This Talk Isn’t About

But worth mentioning anyway:

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The Zen of Predictive ModellingPr

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• The Most Important Part of My “Predictive Modelling and Data Mining Course”

• What every user of predictive modelling should know

• What every manager and owner of predictive modelling capability must know

• “Open Secrets” known to the masters

The Zen of Predictive ModellingPr

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• To save people time

• To see the forest for the trees

• To real value out of predictive analytics

The Right Point of ViewPr

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Which is unlike the other two?

• Kohonen neural network

• Backpropagation neural network

• CART decision tree


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• CART decision tree

• Random Forest

• Support Vector Machine


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• Backpropagation Neural Network

• Linear Model

• CART Decision Tree


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• Out Of Sample Accuracy

• Robustness (Out of Time Accuracy)

• Interpretability

• Implementability


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Why build predictive models ?

• Insights

• Operational prediction

• “What-if” analysis

What Do All Predictive Models Have in Common ?Pr

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All Predictive Models:

• Have a training set of predictors and outcomes

• Probably have a cross-validation and test set of predictors and outcomes too.

• Are “fit” (optimsied) to minimise an error function between their actual and target

outcomes

• Are probably cross-validated to control overfitting on an out-of-sample data set

• Provide information on the relationship between the predictors and outcomes in

the data

• Can be used to score new data (make new predictions)

• Can be deployed in IT systems

• Can be interrogated for insights

• Are only as accurate as the data allows

• Provide a (fairly) accurate estimate of how well they will predict on new data

What Do All Predictive Model Insights Have in Common ?Pr

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• Have variable importance measures (a number of which can be applied to any

model)

• Allow plotting predictors vs outcomes

• Have variable accuracy measures

• Can be resampled for more robust measures of accuracy

What Do All Predictive Model Predictions Have in Common?Pr

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• Make predictions that are numeric : estimates of amount for regression, and

probability for classification

• All predictions are applications of the underlying model structure and parameters

(formula) to new predictor data sets

• All predictions are deterministic. Once a model is fitted, the predictions for a given

record will be the same every time. (Though the prediction may be a distribution

rather than a fixed point. Also, note that model fitting itself may be random – some

models may differ slightly each time they are fitted to the same data set)

How Do Predictive Model Families Differ?Pr

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• Classification vs Regression (most families can do both)

• Predictive accuracy vs insights

• Predictive accuracy vs stability

• Deterministic fitting vs randomised fitting

• Specific insights

• Structure and complexity

• Model assumptions (linear models, neural nets)

• Model structure (trees vs additive models vs SVM vs Neural Nets etc)

• The kinds of insights models provide

• Tendency to overfit (most, but not all)

• Dependence on metrics

• Sensitivity to missing values and categorical variables

Becoming a Master of Modelling Kung FuPr

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• Predictive models should be thought of as a “black box” initially, with the

characteristics that all models have in common recognised

• The focus should be on the data, not the model.

• Focusing on the specific characteristics of the model is important when: deciding on

the degree of accuracy desired, and the kinds of insights desired.

• It is good to start by working with one highly accurate, simple to use method

(randomForest is a good choice) and one or two highly interpretable models (rpart

decision trees and (generalised) linear models are good here.

• In fact, you can go a long way with just randomForest alone.

Becoming a Master of Modelling Kung FuPr

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• Master an adequate tool.

• Empty your mind of the tool . It is an illusion.

• Meditate on the data.

Meditating on DataPr

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• Start with a highly accurate, nonparametric model you are comfortable with.

• The accuracy of a highly accuarate method is close to the theoretical limit of

accuracy possible on the data. World class experts may get closer, but not a whole

lot closer.

• So once you build the model, forget about the specific family you used. It is just a

tool.

• Each predictor may provide a unique amount of predictability to the model.

Measure it.

• Each predictor may be masked by other predictors. Be careful.

• Check relationships between data and strongest predictors

Meditating on DataPr

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• There are at least 3 ways that a predictor can be important. They are not the same:

• What is the unique contribution of the predictor to the accuracy of the model ?

• What is the individual predictive power of the predictor alone ?

• How vital is the predictor to the structure of a particular model ?

• The first two are about the data, the third is more about the specific model. Which

is more important ?

The Predictive Modelling Master’s Data Meditation

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• Start with a highly accurate, nonparametric model you are comfortable with.

• The accuracy of a highly accuarate method is close to the theoretical limit of

accuracy possible on the data. World class experts may get closer, but not a whole

lot closer.

• So once you build the model, forget about the specific family you used. It is just a

tool.

• Measure model accuracy on out-of-sample data. Pay attention to any imbalances in

class or data subset accuracy.

• Measure model stability if necessary (it almost always is)

• Measure the importance of all variables, using the three main techniques.

• Measure again, holding some of the main predictors constant

• Measure (visualise) the effects of each predictor

• Build an interpretable model to help tell the story

The Master Sharpens the Sword : Getting More Accuracy

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• There is never enough data

• Some model accuracy can result from trying other model families. Usually not

much, and not the best use of time, though for some reason the favourite activity of

new data miners.

• Some more model accuracy can result from tweaking model parameters. This is

perhaps less of a waste of time, but still not the ideal focus.

• The most dramatic improvement in model accuracy comes from new predictors.

• New predictors may be entirely new data sets, or complex new transformations of

existing data.

• A large, multi-tabular data set may well have information that has not been

captured in the data.

• The most common information of this type involves relations between individual

records. (eg. Time series windows, geographic neighbourhoods or social network

statistics per record)

Illusions On the Path

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• Colossal wastes of time can include

• Trying to find the “right” model family

• Getting stuck in data preprocessing trying to get all the predictors “right”

• Trying to figure out what the targets should be (usually a sign that the business

problem is not well understood)

• Trying to “improve” the model without defining what that means

The Sun Tzu of Modelling: Be Prepared

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• Know what you are modelling and for what purpose.

• Know what your target variable is. You may have more than one.

• Do not hesitate, model with what you have, and add more predictors later.

• Messy data is better than no data

• Use the right error measures

• Know the connection between the model and your business

• Evaluate, interrogate the model accordingly

• Always question the business value of the analysis

• Always be ready to suggest the business use of the analysis

• Don’t assume that the client understands what to do with the model

Strategy and Tactics

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• Why are you (re)building the model?

• If Strategic: what is going to be done with the insights ? By whom ?

• If Operational: what are the key metrics – accuracy, value, deployability?

Questions ?

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The zen of predictive modelling

Technology

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