In information theory, Fano's inequality (also known as the Fano converse and the Fano lemma) relates the average information lost in a noisy channel to the probability of the categorization error. It was derived by Robert Fano in the early 1950s while teaching a Ph.D. seminar in information theory at MIT, and later recorded in his 1961 textbook.

It is used to find a lower bound on the error probability of any decoder as well as the lower bounds for minimax risks in density estimation.

is the conditional entropy,

is the probability of the communication error, and

is the corresponding binary entropy.

The following generalization is due to Ibragimov and Khasminskii (1979), Assouad and Birge (1983).

Let F be a class of densities with a subclass of r + 1 densities ƒ**θ such that for any θ ≠ θ′

Then in the worst case the expected value of error of estimation is bound from below,

where ƒ**n is any density estimator based on a sample of size n.