Editing Gibbs sampling (section)

== Failure modes ==

There are two ways that Gibbs sampling can fail.  The first is when there are islands of high-probability states, with no paths between them.  For example, consider a probability distribution  over 2-bit vectors, where the vectors (0,0) and (1,1) each have probability {{sfrac|1|2}}, but the other two vectors (0,1) and (1,0) have probability zero.  Gibbs sampling will become trapped in one of the two high-probability vectors, and will never reach the other one.  More generally, for any distribution over high-dimensional, real-valued vectors, if two particular elements of the vector are perfectly correlated (or perfectly anti-correlated), those two elements will become stuck, and Gibbs sampling will never be able to change them.

The second problem can happen even when all states have nonzero probability and there is only a single island of high-probability states.  For example, consider a probability distribution over 100-bit vectors, where the all-zeros vector occurs with probability {{sfrac|1|2}}, and all other vectors are equally probable, and so have a probability of <math>\frac{1}{2(2^{100}-1)}</math> each.  If you want to estimate the probability of the zero vector, it would be sufficient to take 100 or 1000 samples from the true distribution.  That would very likely give an answer very close to {{sfrac|1|2}}.  But you would probably have to take more than <math>2^{100}</math> samples from Gibbs sampling to get the same result.  No computer could do this in a lifetime.

This problem occurs no matter how long the burn-in period is.  This is because in the true distribution, the zero vector occurs half the time, and those occurrences are randomly mixed in with the nonzero vectors.  Even a small sample will see both zero and nonzero vectors.  But Gibbs sampling will alternate between returning only the zero vector for long periods (about <math>2^{99}</math> in a row), then only nonzero vectors for long periods (about <math>2^{99}</math> in a row).  Thus convergence to the true distribution is extremely slow, requiring much more than <math>2^{99}</math> steps; taking this many steps is not computationally feasible in a reasonable time period. The slow convergence here can be seen as a consequence of the [[curse of dimensionality]].
A problem like this can be solved by block sampling the entire 100-bit vector at once. (This assumes that the 100-bit vector is part of a larger set of variables.  If this vector is the only thing being sampled, then block sampling is equivalent to not doing Gibbs sampling at all, which by hypothesis would be difficult.)