How to Use Excel's BetaInv Function

Excel's BetaInv function is the inverse function of the specified beta distribution. Therefore, if a probability is given by BetaDist (x,…), then x is given by BetaInv (probability,…). A beta distribution is frequently used in project planning to predict completion times. The following steps will show how to use Excel's BetaInv function.

Instructions

Difficulty: Moderate

1. Learn the syntax of BetaInv. It is BetaInv (probability,alpha,beta,A,B) where probability is the result of the beta distribution, alpha is the alpha parameter of the beta distribution and beta is the beta parameter of the beta distribution. Parameters A and B are optional and will be explained in Step 2.
2. Include A as the lower bound to x and B as the upper bound to x when a non-standard cumulative beta distribution is desired. Otherwise, BetaInv will use the standard cumulative beta distribution where A=0 and B=1.
3. Study the way BetaInv works. BetaInv determines x such that BetaDist(x, alpha, beta, A, B) = p given the value p. The precision of BetaInv is dependent upon the precision of BetaDist. BetaInv returns the #N/A error value if it does not find x after 100 iterations.
4. Observe the error conditions of BetaInv. BetaInv will return the #Value! error value if any argument is non-numeric. BetaInv will return the #Num! error value if alpha <= 0, beta <= 0, p <= 0 or p > 1.
5. Look at an example of BetaInv.
   
   =BetaInv (0.685470581,8,10,1,3) returns 2.
   
   0.685470581 is the probability given by the beta distribution.
   8 is the alpha parameter of the beta distribution.
   10 is the beta parameter of the beta distribution.
   1 is the lower bound of x.
   3 is the upper bound of x.

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