机器学习的三种类型
监督学习，加强学习和非监督学习
Types of learning task
• Supervised learning
– Learn to predict an output when given an input vector.
• Reinforcement learning
– Learn to select an action to maximize payoff.
• Unsupervised learning
– Discover a good internal representation of the input
Two types of supervised learning
•Each training case consists of an input vector x and a target output t.
• Regression: The target output is a real number or a whole vector of real numbers.
– The price of a stock in 6 months time.
– The temperature at noon tomorrow.
• Classification: The target output is a class label.
– The simplest case is a choice between 1 and 0.
– We can also have multiple alternative labels.
How supervised learning typically works
We start by choosing a model-class:
– A model-class, f, is a way of using some numerical parameters, W, to map each input vector, x, into a predicted
output y.
• Learning usually means adjusting the parameters to reduce the discrepancy between the target output, t, on each training case and the actual output, y, produced by the model.
– For regression, is often a sensible measure of the discrepancy.
– For classification there are other measures that are generally more sensible (they also work better).
Reinforcement learning
•In reinforcement learning, the output is an action or sequence of actions and the only supervisory signal is an occasional scalar reward.
– The goal in selecting each action is to maximize the expected sum of the future rewards.
– We usually use a discount factor for delayed rewards so that we don’t have to look too far into the future.
• Reinforcement learning is difficult:
– The rewards are typically delayed so its hard to know where we went wrong (or right).
– A scalar reward does not supply much information.
• This course cannot cover everything and reinforcement learning is one of the important topics we will not cover.
Unsupervised learning
•For about 40 years, unsupervised learning was largely ignored by the machine learning community
– Some widely used definitions of machine learning actually excluded it.
– Many researchers thought that clustering was the only form of unsupervised learning.
• It is hard to say what the aim of unsupervised learning is.
– One major aim is to create an internal representation of the input that is useful for subsequent supervised or reinforcement learning.
– You can compute the distance to a surface by using the disparity between two images. But you don’t want to learn to compute disparities by stubbing your toe thousands of times.
Other goals for unsupervised learning
• It provides a compact, low-dimensional representation of the input.
– High-dimensional inputs typically live on or near a low dimensional manifold (or several such manifolds).
– Principal Component Analysis is a widely used linear method for finding a low-dimensional representation.
• It provides an economical high-dimensional representation of the input in terms of learned features.
– Binary features are economical.
– So are real-valued features that are nearly all zero.
• It finds sensible clusters in the input.
– This is an example of a very sparse code in which only one of the features is non-zero.