案例学习：预测房价

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From the course by University of Washington

机器学习：回归

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案例学习：预测房价

From the lesson

Feature Selection & Lasso

A fundamental machine learning task is to select amongst a set of features to include in a model. In this module, you will explore this idea in the context of multiple regression, and describe how such feature selection is important for both interpretability and efficiency of forming predictions. <p> To start, you will examine methods that search over an enumeration of models including different subsets of features. You will analyze both exhaustive search and greedy algorithms. Then, instead of an explicit enumeration, we turn to Lasso regression, which implicitly performs feature selection in a manner akin to ridge regression: A complex model is fit based on a measure of fit to the training data plus a measure of overfitting different than that used in ridge. This lasso method has had impact in numerous applied domains, and the ideas behind the method have fundamentally changed machine learning and statistics. You will also implement a coordinate descent algorithm for fitting a Lasso model. <p>Coordinate descent is another, general, optimization technique, which is useful in many areas of machine learning.

- Emily FoxAmazon Professor of Machine Learning

Statistics - Carlos GuestrinAmazon Professor of Machine Learning

Computer Science and Engineering

[MUSIC]

Well, this all subsets algorithm might seem great or

at least pretty straight forward to implement, but

a question is what's the complexity of running all subsets?

How many models did we have to evaluate?

Well, clearly what we evaluated all models, but let's quantify what that is.

So we looked at the model that was just noise.

We looked at the model with just the first feature, second feature, all the way up to

the model with just the first two features, the second two features and

every possible model up to the full model of all D features.

And what we can do is we can index each one of these models that we searched over

by a feature vector.

And this feature vector is going to say, so for

feature one, feature two, all the way up to feature D,

what we're gonna enter is zero if no,

that feature is not in the model, and one if yes, that feature is in the model.

So it's just gonna be a binary vector indicating which features are present.

So, in the case of just noise, or

no features we have zeros along this whole vector.

In the case of just the first model,

I made the first feature be included in the model, we're just gonna have a one

in that first feature location, and zeros everywhere else.

I guess for consistency, let me index this as feature zero, feature one.

All the way up to feature D.

Okay, and we're gonna go through this entire set of possible feature vectors,

and how many choices are there for the first entry, two.

How many for the second, two, two, two choices for every entry.

And how many entries are there?

Well, with my new indexing, instead of D there's really D plus one.

That's just a little notational choice.

And I did a little back of the envelope calculation for a couple choices of D.

So for example, if we had a total of eight different features we were looking over,

then we would have to search over 256 models.

That actually might be okay.

But if we had 30 features, all of a sudden we have to search over 1

billion some number of different models.

And if we have 1,000 features,

which really is not that many in applications we look at these days,

all of a sudden we have to search over 1.07 times 10 to the 301.

And for the example I gave with 100 billion features,

I don't even know what that number is.

Well, I'm sure I could go and compute it, but

I didn't bother and it's clearly just huge.

So, what we can see is that typically, and

in most situations we're faced with these days.

It's just computationally prohibitive to do this all subset search.

[MUSIC]

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