# P-value for multinomial distribution

I am testing the significance of observed data sampled ($n\approx50$) using a multinomial distribution with known probabilities (with ~20 categories). Given the probability of observing the sample $P_{sam}$, I would like to compute the p-value, which is defined as the sum of all probabilities lower than this observed $P_{sam}$ i.e. $Sum\{p \mid p < P_{sam}, p \in Multinomial-distribution\}$.

One way to find this is to generate a large number of random samples from the given distribution and calculate the above sum using this set. But I am wondering if there exist any exact results and whether there are any good approximations which are less computationally intensive.

Thanks!

• The $p$-value is always related to a hypothesis test. What is the null hypothesis ? Commented Oct 9, 2012 at 20:49
• Sorry for the late reply, but the null hypothesis is that the observed samples are drawn from a multinomial distribution characterized by known frequencies of ~20 categories. Commented Mar 17, 2013 at 22:17

You say you know the probability of each side of the die and there are about 20 sides. Let's denote the sides $i$ for $i = 1, 2, ..., 20$. So your answer is exactly the definition-- $p$-value$(i_0) =$ $$\sum_{ \{ i : p(i) \leq p(i_0)\} } p(i).$$
Note that I disagree slightly with your definition. I think the $<$ in your sum should be $\leq$.