# Estimating distribution parameters from few data points

Say I'm doing stats on the height of adults from various countries.

I assume the heights of adults from one country are normally distributed, and ignore sex differences (I also ignore the fact that neighbouring countries tend to have similar populations).

I have a bunch of data by country, but for some countries I have very few data points, which can lead to quite big errors on my estimate of standard distributions.

Is there a way I can use the data from countries for which I have a lot of data to get better estimates - say if I notice standard deviation in those countries is always between 7 and 8.5 cm, but my dataset for Nepal (for which I have 9 samples) has a standard deviation of 9.5 cm, I should probably correct that downwards. But how? Is there a formula for this?

When calculating the parameters for Nepal, shouldn't the data from the other countries allow me to have a "prior" distribution of expected means and deviations, which I would then update by taking the actual data from Nepal into account? How would I formalize this methodology?

(I got this while looking for a simple reduction of the problem that prompted previous question, which didn't get an answer yet - I'm still mostly looking for good methodologies for thinking about this kind of problem).

-
 About using priors, I would rather suggest to look if there are some data available for country with small $n$ at the population level (e.g., through census or national surveys). – chl♦ Dec 6 '10 at 12:41 chl: Actually I'm not really studying data on people's height, I just took that as a simplified example. My actual data has much more dimensions. – Emile Dec 7 '10 at 10:12

## 2 Answers

The correct answer here is hierarchical modeling (also called multilevel modeling). What you want to do is have the variance parameters drawn from a common prior distribution who's parameters are also estimated. Something like

CountryVariance_i ~ D(Location, Scale)

Location ~ D_2(LocationPriorParams)

Scale ~ D_3(ScalePriorParams)

Where D, D_2, D_3 are whatever distributions would like.

This formalizes the notion that the variance parameters should be similar, and will do the correct kind of shrinkage. I believe that Gelman's books (Bayesian Data Analysis and Data Analysis Using Regression and Multilevel/Hierarchical Models) talk quite a bit about this kind of thing (though perhaps not a lot for variance parameters).

I don't have a great introduction on hand, so I recommend looking at the wikipedia page and searching for "Hierarchical modeling" or something and clicking on various introductions (there are many). Look for graphs as these are helpful for understanding hierarchical models.

-

What you seem to be referring to is called "shrinkage". This allows you to share strength across groups and is frequently used in hierarchical Bayesian models. The (very) basic idea is to impose a prior distribution over the entire population and place more weight on the prior for those groups that have few samples, much like you would for any Bayesian analysis.

The best reference I can think of for this is ch. 8 of Peter Hoff's book: "A First Course in Bayesian Statistical Methods", but if you don't have access to this book, I'm sure any book on Bayesian statistics will talk about this (search for "group comparison", "hierarchical modeling", or "shrinkage"). I know that Robert Gramacy has done some work on this as well, so you may wish to check out his publications.

-