Syntax
GAMMADIST(x, alpha, beta, cumulative)
| Parameter | Description |
|---|---|
| x | Parameter of the GAMMADIST function. |
| alpha | Parameter of the GAMMADIST function. |
| beta | Parameter of the GAMMADIST function. |
| cumulative | Parameter of the GAMMADIST function. |
Examples
Rainfall probability
Formula
=GAMMADIST(50, 2, 20, TRUE)
Returns ~0.7127. About 71% of months have 50mm or less rainfall under this gamma rainfall model with alpha=2, beta=20.
Component lifetime
Formula
=GAMMADIST(1000, 5, 200, TRUE)
Returns ~0.5595. About 56% of components fail within 1000 hours in this reliability model.
Density at the mode
Formula
=GAMMADIST(2, 3, 1, FALSE)
Returns ~0.2707. The PDF value at x=2 for shape=3, scale=1. The mode of this distribution is at (alpha-1)*beta = 2.
Common Errors
#NUM!
x must be non-negative, alpha and beta must be positive.
#VALUE!
Occurs when arguments are non-numeric.
Tips
Use GAMMA.DIST instead
GAMMADIST is the compatibility version. GAMMA.DIST is the modern replacement with identical functionality.
Fitting data to gamma
To fit a gamma distribution to data, estimate alpha = (mean/std)^2 and beta = variance/mean. Then use GAMMADIST with these parameters.
Always right-skewed
The gamma distribution is always positively skewed (right-skewed). It becomes more symmetric as alpha increases.
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