# Three Parameter Gamma Distribution

What is the motivation for the three parameter gamma distribution and the resulting structure of its density?

What is the meaning of the location, scale and shape parameters here?

• Which three parameter gamma? I've seen more than one. Do you mean the one that's simply a shifted two parameter gamma? – Glen_b -Reinstate Monica Oct 18 '13 at 19:16
• The meaning of "location parameter" and "scale parameter" is independent of the distribution. Also, what do you mean by "structure of its density"? – jbowman Oct 19 '13 at 0:00
• It refers to the probability density function – Vani Oct 19 '13 at 4:42
• @Carl That must be very confusing for physicists when they want to talk about a function that represents actual mass-density. I can understand a statistician dropping the adjective, but for a physicist it would seem to be necessary to keep it, since they might regularly need to refer to more than one kind of density. – Glen_b -Reinstate Monica Dec 19 '16 at 0:28
• I wouldn't call a pdf a probability function. It's a density function, but since more than one thing can have density, we need to be able to express what it's a density of (and that's probability). The only think I'd normally call a probability function is the function that represents P(X=x) for a discrete random variable. – Glen_b -Reinstate Monica Dec 19 '16 at 0:55

If you need to shift the distribution, that's where you use the three-parameters gamma distribution. In this instance, the mean of the distribution is simply $\text{mean}=\text{location}+\text{shape}\cdot \text{scale}$.