**0**wrote:

Hello all, I have another question for the experts where I did not find any answer, yet. It's about the shape parameter of the gamma distribution of rate variation. Known as Alpha. I know what the parameter is, and I know that if when Alpha = 1 the gamma distribution matches the exponential distribution. My question is: What does the setting "exponential" for example at MrBayes mean? At MrBayes I can set Shapepr=exponential(value). And I can set Shapepr=fixed(value). What is the difference? All the best Dieter

**2.7k**• written 6 days ago by dieter •

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See this pdf-file for details of gamma-distribution:

https://www.google.ru/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=0ahUKEwiypbSgvfPXAhXKHJoKHQWxCakQFghtMAg&url=http%3A%2F%2Fmath.bme.hu%2F~nandori%2FVirtual_lab%2Fstat%2Fspecial%2FGamma.pdf&usg=AOvVaw0cPMyeFnsVsX0wWHuD4ruJ

This Biostar posts on MrBayes and gamma distibution:

The Effect Of The "Invgamma" Setting On Computing The Site Likelihood In Mrbayes?

and ask stackoverflow if the above ones are not enough:

https://stackoverflow.com/questions/29902770/how-to-specify-gamma-distribution-using-shape-and-rate-in-python

2.7kThanks Natasha, But I'm sorry, the pdf only describes the gamma distribution (which I already know) - all is clear there - but it does not answr my questions. The second link leads to a discussion about the Invar Model - this has nothing to do with my question. But thanks anyway. All the best Dieter

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