How Do You Calculate Probabilities in a Standard Gamma Distribution?

[kdizzle711]

Homework Statement

This is an example in my book with omitted steps. Suppose the reaction time X of a randomnly selected individual to a certain stimulus has a standard gamma distribution with alpha=2. When X is continuous

P(2<=X<=5) = F(5;2)-F(3;2) = .960-.801 = .159

Homework Equations

F(x;alpha) = integral from x to 0 [(y^(alpha-1))*(e^-y)]/(gamma(alpha)

The Attempt at a Solution

I have followed the equation and placed 5 for y and 2 for alpha but the numbers are not matching up. In the test it says that gamma(alpha) is equal to one. Please help I have a test tommorow. Thanks

 

[rasmhop]

What exactly is the problem? What numbers are you expecting and what numbers are you getting?

You write:
P(2 <= X <= 5) = .159
but is this what you're supposed to get, or what you're actually getting?

I get P(2 <= X <=5)=.159 as well using the exact formulas you posted (minor nitpick: the integral is from 0 to x, not from x to 0, but I assume this was just a typo).