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Why use a bias/threshold?...

Author Message
Jens Burmeister...
Posted: Thu Sep 17, 2009 6:30 pm
Guest
I've read the infos in the FAQ about the mentioned topic
(<ftp://ftp.sas.com/pub/neural/FAQ2.html#A_bias>) but I don't understand it
all...

What's the reason for using BIAS/threshold. What is the advantage compared
with omitting the Bias?

I have understood that the hyperplanes always pass through the origin
without a bias. Is that only an disadvantage for learning or can there be a
situation, where learning would be impossible without bias-neuron.

(We're discussing that topic in our workgroup and no one knows a good
answer, why we use the bias-neurons, but everybody does... so, I'm hoping to
get the answer here)

regards
Jens
 
Greg...
Posted: Fri Sep 18, 2009 1:18 am
Guest
On Sep 17, 10:30 am, "Jens Burmeister" <st... at (no spam) jensburmeister.de>
wrote:
Quote:
I've read the infos in the FAQ about the mentioned topic
(<ftp://ftp.sas.com/pub/neural/FAQ2.html#A_bias>) but I don't understand it
all...

What's the reason for using BIAS/threshold. What is the advantage compared
with omitting the Bias?

If I ask you to fit a line to some arbitrary (x,y) data which
model would you use?

y = m*x

or

y = m*x + b?

Hope this helps.

Greg
 
Jens Burmeister...
Posted: Fri Sep 25, 2009 10:38 am
Guest
Quote:
How can you do without a bias? I.e. with a (tilted) plane passing through
the origin?

I understood your example well. But when I have a hidden layer with a number
of neurons (let's say 10), I thought, that it is possible to solve that
separation problem, because with the hidden layer, I have a number of
degrees of freedom with which I can separate. Is that wrong?
 
Jonathan Campbell...
Posted: Fri Sep 25, 2009 9:18 pm
Guest
Jens Burmeister wrote:
Quote:
How can you do without a bias? I.e. with a (tilted) plane passing through
the origin?

I understood your example well. But when I have a hidden layer with a number
of neurons (let's say 10), I thought, that it is possible to solve that
separation problem, because with the hidden layer, I have a number of
degrees of freedom with which I can separate.

Perhaps, but I can think about only one neuron at a time and I would
like to give all neurons' hyperplanes as much freedom as possible.
Moreover, I would really hate to use a hidden layer when one was not
necessary --- all for the want of a bias input.

If you look at section 4.2 of:

http://www.jgcampbell.com/ip/pr.pdf

you will see an arm-wavy construction that shows that a p-h-1 network
can implement an arbitrarily complex decision region (not my idea,
originally in a Lippman paper, I think).

Best regards,

Jon C.


--
Jonathan Campbell www.jgcampbell.com BT48, UK.
 
Greg...
Posted: Sat Sep 26, 2009 7:18 pm
Guest
On Sep 25, 1:18 pm, Jonathan Campbell <jg.campbell... at (no spam) gmail.com>
wrote:
Quote:
Jens Burmeister wrote:
How can you do without a bias? I.e. with a (tilted) plane passing through
the origin?

I understood your example well. But when I have a hidden layer with a number
of neurons (let's say 10), I thought, that it is possible to solve that
separation problem, because with the hidden layer, I have a number of
degrees of freedom with which I can separate.

Perhaps, but I can think about only one neuron at a time and I would
like to give all neurons' hyperplanes as much freedom as possible.
Moreover, I would really hate to use a hidden layer when one was not
necessary --- all for the want of a bias input.

If you look at section 4.2 of:

http://www.jgcampbell.com/ip/pr.pdf

you will see an arm-wavy construction that shows that a p-h-1 network
can implement an arbitrarily complex decision region (not my idea,
originally in a Lippman paper, I think).

There are more authoratative references in the
12 message 1991 thread "IS ONE HIDDEN LAYER
ALWAYS ENOUGH?" containing the post

http://groups.google.com/group/comp.ai.neural-nets/
msg/941a369762476bf9?hl=en


Hope this helps,

Greg
 
 
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