[EMAIL PROTECTED] wrote:
4. Since the chi-squared and exponential distributions are just special cases of the gamma distribution, there is no need to have separate implementation classes for these. In my opinion, one should avoid having multiple implementations of the same distribution unless there is some strong reason for it.
Please keep in mind that not every user is a full-blown statistician knowing all these underlying interactions. For that reason, I would argue strongly against this.
It wouldn't make any difference to the end user as I guess he's supposed to use the DistributionFactory to create the distributions he need.
You could keep the ExponentialDistribution and ChiSquaredDistribution interfaces and just change the following in DistributionFactoryImpl:
public ChiSquaredDistribution createChiSquareDistribution(
final double degreesOfFreedom) {
return new GammaDistributionImpl(degreesOfFreedom/2, 2);
}
public ExponentialDistribution createExponentialDistribution(double mean) {
return new GammaDistributionImpl(1, mean);
}
Very Bad Idea, this means that GammaDistribution would have to implement Exponential and ChiSquare interfaces, if the "beta scale parameter" exposed in the Gamma interface were changed then this would be exceptionally confusing to the user. They could in a sense make a ChiSquare instance into some other distribution by changing the properties available on the class. This is what the default implementation of ChiSquareDist should be enforcing, if it is not, then the interface should be further restricted.
Then you can drop ExponentialDistributionImpl and ChiSquaredDistributionImpl altogether.
The way to do this is to maintain ExponentialDistributionImpl and ChiSquaredDistributionImpl as "wrappers" that just lockdown GammaDistribution to a specific configuration (via special Constructor and overriding the api to not allow the "beta scale parameter" to change).
This is the point of these interfaces/implementations. Yes, everyone could learn that these are all just variations on a Gamma distribution, I learned that in my college statistics class. But providing common default implementations is important for ease of use.
-Mark
-- Mark Diggory Software Developer Harvard MIT Data Center http://www.hmdc.harvard.edu
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