1.4 Machine learning lecture 5  (Page 22/15)

MachineLearning-Lecture05

Instructor (Andrew Ng) :Okay, good morning. Just one quick announcement and reminder, the project guidelines handout was posted on the course website last week. So if you haven’t yet downloaded it and looked at it, please do so. It just contains the guidelines for the project proposal and the project milestone, and the final project presentation.

So what I want to do today is talk about a different type of learning algorithm, and, in particular, start to talk about generative learning algorithms and the specific algorithm called Gaussian Discriminant Analysis. Take a slight digression, talk about Gaussians, and I’ll briefly discuss generative versus discriminative learning algorithms, and then hopefully wrap up today’s lecture with a discussion of Naive Bayes and the Laplace Smoothing.

So just to motivate our discussion on generative learning algorithms, right, so by way of contrast, the source of classification algorithms we’ve been talking about I think of algorithms that do this. So you’re given a training set, and if you run an algorithm right, we just see progression on those training sets.

The way I think of logistic regression is that it’s trying to find – look at the date and is trying to find a straight line to divide the crosses and O’s, right? So it’s, sort of, trying to find a straight line. Let me – just make the days a bit noisier. Trying to find a straight line that separates out the positive and the negative classes as well as pass the law, right?

And, in fact, it shows it on the laptop. Maybe just use the screens or the small monitors for this. In fact, you can see there’s the data set with logistic regression, and so I’ve initialized the parameters randomly, and so logistic regression is, kind of, the outputting – it’s the, kind of, hypothesis that iteration zero is that straight line shown in the bottom right.

And so after one iteration and creating descent, the straight line changes a bit. After two iterations, three, four, until logistic regression converges and has found the straight line that, more or less, separates the positive and negative class, okay? So you can think of this as logistic regression, sort of, searching for a line that separates the positive and the negative classes.

What I want to do today is talk about an algorithm that does something slightly different, and to motivate us, let’s use our old example of trying to classify the team malignant cancer and benign cancer, right? So a patient comes in and they have a cancer, you want to know if it’s a malignant or a harmful cancer, or if it’s a benign, meaning a harmless cancer.

So rather than trying to find the straight line to separate the two classes, here’s something else we could do. We can go from our training set and look at all the cases of malignant cancers, go through, you know, look for our training set for all the positive examples of malignant cancers, and we can then build a model for what malignant cancer looks like. Then we’ll go for our training set again and take out all of the examples of benign cancers, and then we’ll build a model for what benign cancers look like, okay?