  Calculating the Shannon entropy for symbolic sequences has been widely
considered in many fields. For descriptive statistical problems such as
estimating the N-gram entropy of English language text, a common approach is to
use as much data as possible to obtain progressively more accurate estimates.
However in some instances, only short sequences may be available. This gives
rise to the question of how many samples are needed to compute entropy. In this
paper, we examine this problem and propose a method for estimating the number
of samples required to compute Shannon entropy for a set of ranked symbolic
natural events. The result is developed using a modified Zipf-Mandelbrot law
and the Dvoretzky-Kiefer-Wolfowitz inequality, and we propose an algorithm
which yields an estimate for the minimum number of samples required to obtain
an estimate of entropy with a given confidence level and degree of accuracy.
