LFD Book Forum lecture 8: understanding bias

#1
02-09-2013, 09:41 AM
 ilya239 Senior Member Join Date: Jul 2012 Posts: 58
lecture 8: understanding bias

The VC dimension is single number that is a property of the hypothesis set.
But, what is "bias of a hypothesis set"? Bias seems to depend also on dataset size and the learning algorithm, since it depends on ; depends on the learning algorithm, and the set of datasets over which the expectation is taken depends on dataset size.

Slide 4 says that bias measures "how well can approximate ". Does this mean "with a sufficiently large dataset and a perfect learning algorithm"?
Is the bias of a (hypothesis set, learning algorithm) combination a single value -- the asymptote of the learning curve? Or is there some notion of bias that is a property of a hypothesis set by itself? If the hypothesis set contains the target function, that does not mean the bias is zero, does it? The beginning of the lecture seems to imply otherwise, but if there is no restriction on the learning algorithm, what guarantees that the average function will in fact be close to the target function for large enough dataset size?
Or is it assumed that the learning algorithm always picks a hypothesis which minimizes ?
#2
02-09-2013, 01:01 PM
 yaser Caltech Join Date: Aug 2009 Location: Pasadena, California, USA Posts: 1,477
Re: lecture 8: understanding bias

Quote:
 Originally Posted by ilya239 The VC dimension is single number that is a property of the hypothesis set. But, what is "bias of a hypothesis set"? Bias seems to depend also on dataset size and the learning algorithm, since it depends on ; depends on the learning algorithm, and the set of datasets over which the expectation is taken depends on dataset size.
Your observation is correct that the bias-variance analysis is not as general as the VC analysis. The bias does depend on the learning algorithm. It also depends on the number of examples, usually slightly.

Quote:
 Slide 4 says that bias measures "how well can approximate ". Does this mean "with a sufficiently large dataset and a perfect learning algorithm"? Is the bias of a (hypothesis set, learning algorithm) combination a single value -- the asymptote of the learning curve? Or is there some notion of bias that is a property of a hypothesis set by itself? If the hypothesis set contains the target function, that does not mean the bias is zero, does it? The beginning of the lecture seems to imply otherwise, but if there is no restriction on the learning algorithm, what guarantees that the average function will in fact be close to the target function for large enough dataset size? Or is it assumed that the learning algorithm always picks a hypothesis which minimizes ?
Good questions . What you are saying would hold if we were using the best approximation of in as the vehicle for measuring the bias. We are not. We are using a "limited resource" version of it that is based on averaging hypotheses that we get from training on a finite set of data points. This version is often close to the best approximation so that's why we can take that liberty.
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#3
02-09-2013, 02:31 PM
 ilya239 Senior Member Join Date: Jul 2012 Posts: 58
Re: lecture 8: understanding bias

Quote:
 Originally Posted by yaser The bias does depend on the learning algorithm. It also depends on the number of examples, usually slightly. ... This version is often close to the best approximation so that's why we can take that liberty.
Thanks for the explanation.
In HW4 #4 the average hypothesis is measurably shifted from the hypothesis set member giving the lowest mean squared error. Probably because two-point dataset is too small, i.e. this is not representative of realistic cases?
#4
02-09-2013, 06:49 PM
 yaser Caltech Join Date: Aug 2009 Location: Pasadena, California, USA Posts: 1,477
Re: lecture 8: understanding bias

Quote:
 Originally Posted by ilya239 Thanks for the explanation. In HW4 #4 the average hypothesis is measurably shifted from the hypothesis set member giving the lowest mean squared error. Probably because two-point dataset is too small, i.e. this is not representative of realistic cases?
Indeed, the fewer the number of points, the more likely that the average hypothesis will differ from the best approximation. The difference tends to be small, though.
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#5
02-11-2013, 10:50 AM
 gah44 Invited Guest Join Date: Jul 2012 Location: Seattle, WA Posts: 153
Re: lecture 8: understanding bias

Quote:
 Originally Posted by ilya239 Thanks for the explanation. In HW4 #4 the average hypothesis is measurably shifted from the hypothesis set member giving the lowest mean squared error. Probably because two-point dataset is too small, i.e. this is not representative of realistic cases?
Well, it is also that the two point data set is small relative to the two parameter hypotheses. If you have 100 points, and 99th degree polynomials, it would also have large variance. I will guess that minimizing bias plus variance happens with the number of fit parameters near the square root of the number of points per data set.
#6
02-11-2013, 11:45 AM
 ilya239 Senior Member Join Date: Jul 2012 Posts: 58
Re: lecture 8: understanding bias

Quote:
 Originally Posted by gah44 Well, it is also that the two point data set is small relative to the two parameter hypotheses. If you have 100 points, and 99th degree polynomials, it would also have large variance. I will guess that minimizing bias plus variance happens with the number of fit parameters near the square root of the number of points per data set.
Large variance, sure. I was trying to understand why large bias. If you take a huge number of 100-point datasets, learn a hypothesis from each, and take the average value of these, why might it be far from the target function's value?
On the other hand, I'm not sure how to prove that it won't be far
#7
02-11-2013, 01:04 PM
 yaser Caltech Join Date: Aug 2009 Location: Pasadena, California, USA Posts: 1,477
Re: lecture 8: understanding bias

Quote:
 Originally Posted by ilya239 I was trying to understand why large bias. If you take a huge number of 100-point datasets, learn a hypothesis from each, and take the average value of these, why might it be far from the target function's value? On the other hand, I'm not sure how to prove that it won't be far
It is unlikely (as a practical observation) to be far, but it is likely to be different.
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