 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
• |
Directed and
undirected models seem very different.
|
|
|
• |
But there is a
special type of multi-layer directed model
|
|
|
in which it is
easy to infer the posterior distribution over
|
|
|
the hidden units
because it has complementary
priors.
|
|
|
• |
This special
type of directed model is equivalent to an
|
|
|
undirected model.
|
|
|
|
– |
At
first, this equivalence just seems like a neat trick
|
|
|
|
– |
But
it leads to a very effective new learning algorithm
|
|
that
allows multilayer directed nets to be learned one
|
|
|
layer
at a time.
|
|
|
|
• |
The new
learning algorithm resembles boosting with each
|
|
|
layer
being like a weak learner.
|
|