- Mar 5, 2013
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Predictive analytics is just a bunch of math, isn’t it? After all, algorithms in the form of matrix algebra, summations, integrals, multiplies and adds are the core of what predictive modeling algorithms do. Even rule-based approaches need math to compute how good the if-then-else rules are.

I was participating in a predictive analytics course recently and the question a participant asked at the end of two days of instruction was this: “it’s been a long time since I’ve had to do this kind of math and I’m a bit rusty. Is there a book that would help me learn the techniques without the math?”

The question about math was interesting. But do we need to know the math to build models well? Anyone can build a bad model, but to build a good model, don’t we need to know what the algorithms are doing? The answer, of course, depends on the role of the analyst. I contend, however, that for most predictive analytics projects, the answer is “no”.

Let’s consider building decision tree models. What options does one need to set to build good trees? Here is a short list of common knobs that can be set by most predictive analytics software packages:

- Splitting metric (CART style trees, C5 style trees, CHAID style trees, etc.)
- Terminal node minimum size
- Parent node minimum size
- Maximum tree depth
- Pruning options (standard error, Chi-square test p-value threshold, etc.)

The most mathematical of these knobs is the splitting metric. CART-styled trees use the Gini Index, C5 trees use Entropy (information gain), and CHAID style trees use the chi-square test as the splitting criterion. A book I consider the best technical book on data mining and statistical learning methods, “The Elements of Statistical Learning”, has this description of the splitting criteria for decision trees, including the Gini Index and Entropy:

To a mathematician, these make sense. But without a mathematics background, these equations will be at best opaque and at worst incomprehensible. (And these are not very complicated. Technical textbooks and papers describing machine learning algorithms can be quite difficult even for more seasoned, but out-of-practice mathematicians to understand).

As someone with a mathematics background and a predictive modeler, I must say that the actual splitting equations almost never matter to me. Gini and Entropy often produce the same splits or at least similar splits. CHAID differs more, especially in how it creates multi-way splits.

There are, however, very important reasons for someone on the team to understand the mathematics or at least the way these algorithms work qualitatively. First and foremost, understanding the algorithms helps us uncover why models go wrong. Models can be biased toward splitting on particular variables or even particular records. In some cases, it may appear that the models are performing well but in actuality they are brittle. Understanding the math can help remind us that this may happen and why.

The fact that linear regression uses a quadratic cost function tells us that outliers affect overall error disproportionately. Understanding how decision trees measure differences between the parent population and sub-populations informs us why a high-cardinality variable may be showing up at the top of our tree, and why additional penalties may be in order to reduce this bias.

The answer to the question if predictive modelers need to know math is this: no they don’t need to understand the mathematical notation, but neither should they ignore the mathematics. Instead, we all need to understand the effects of the mathematics on the algorithms we use. “Those who ignore statistics are condemned to reinvent it,” warns Bradley Efron of Stanford University. The same applies to mathematics.

*Dean Abbott is President of Abbott Analytics in San Diego, California. Mr. Abbott has over 21 years of experience applying advanced data mining, data preparation, and data visualization methods in real-world data intensive problems, including fraud detection, risk modeling, text mining, response modeling, survey analysis, planned giving, and predictive toxicology. In addition, Mr. Abbott serves as chief technology officer and mentor for start-up companies focused on applying advanced analytics in their consulting practices.*

*Mr. Abbott is a seasoned instructor, having taught a wide range of data mining tutorials and seminars for a decade to audiences of up to 400, including PAW, KDD, AAAI, IEEE and several data mining software users conferences. He is the instructor of well-regarded data mining courses, explaining concepts in language readily understood by a wide range of audiences, including analytics novices, data analysts, statisticians, and business professionals. Mr. Abbott also has taught applied data mining courses for major software vendors, including SPSS-IBM Modeler (formerly Clementine), Unica PredictiveInsight (formerly Affinium Model), Enterprise Miner (SAS), Model 1 (Group1 Software), and hands-on courses using Statistica (Statsoft), Tibco Spotfire Miner (formerly Insightful Miner), and CART (Salford Systems).*

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