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The purpose of this article is to describe the BETAINV function in Microsoft Office Excel 2003 and in Microsoft Office Excel 2007. This article also discusses an improvement in Excel 2003 and in Excel 2007 that might affect the results of the BETAINV function for Excel 2003 and for Excel 2007 when compared to the results of the BETAINV function in earlier versions of Excel.
Note In this article, p is a probability with 0 < p < 1, alpha and beta are positive numbers, and A and B are numbers with A < B. If A and B are omitted, they are assumed to be 0 and 1, respectively.
The BETAINV(p, alpha, beta, A, B) function is the inverse function for BETADIST(x, alpha, beta, A, B). For any particular x, BETADIST(x, alpha, beta, A, B) returns the probability that a Beta-distributed random variable (with the parameters alpha, beta, A, and B) is less than or equal to x. In other words, BETADIST returns the cumulative probability that is associated with x. If A and B are removed, they are assumed to be 0 and 1, respectively.
The BETAINV(p, alpha, beta, A, B) function returns the value of x where BETADIST(x, alpha, beta, A, B) returns p. Therefore, BETAINV is evaluated by a search process that returns the appropriate value of x by evaluating BETADIST for various candidate values of x until it finds a value of x where BETADIST(x, alpha, beta, A, B) is "acceptably close" to p.
Example UsageTo illustrate BETAINV, follow these steps:
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The first two columns of the worksheet illustrate the standard Beta distribution with the optional parameters A and B omitted. You can investigate different parameter settings by changing the values in cells B1:B2. The standard Beta distribution has values between 0 and 1. Cells A5:A8 give the cumulative probability of an observed value that is less than or equal to 0.2, 0.4, 0.6, and 0.8. Cells B5:B8 verify the inverse relationship between BETADIST and BETAINV. Depending on the formatting of cells B5:B8, the values that are returned may or may not be exactly equal to 0.2, 0.4, 0.6, and 0.8, respectively. These numbers are considered equal for practical purposes (for example, the numbers agree to at least 5 decimal places).
Cells A10:A18 use BETAINV to show various cutoff points of the standard Beta distribution cumulative. You can investigate the shape of the Beta distribution cumulative for various values of alpha and beta by changing the entries in cells B1:B2.
The non-standard Beta distribution case is illustrated in cells C5:C8 where A = 10 and B = 20. The standard Beta distribution is basically shifted to the right so that its lower endpoint is at 10 instead of at 0. It is also spread out so that probability fills an interval of width 10 (from 10 to 20) instead of width 1 (from 0 to 1). You can see how the results of BETAINV in cells C5:C8 relate to those in cells B5:B8. In cell B5, 0.2 is 0 + 0.2 * 1, where 0 and 1 are the left endpoint and width (respectively) of the range of the standard Beta distribution. In cell C5, 12 is 10 + 0.2 * 10, where 10 and 10 are the left endpoint and width of the range (that is, B – A) of the Beta distribution when A = 10 and B = 20. Similar relationships exist between corresponding entries in cells B6:B8 and cells C6:C8.
Results in Earlier Versions of ExcelBETAINV(p, alpha, beta, A, B) is found through an iterative process that repeatedly evaluates BETADIST(x, alpha, beta, A, B) and returns a value of x such that BETADIST(x, alpha, beta, A, B) is "acceptably close" to p. Therefore, the accuracy of BETAINV depends on the following two factors:
Results in Excel 2003 and in Excel 2007No changes were made in Excel 2003 and in Excel 2007 to BETADIST. The only change that affects BETAINV is that "acceptably close" is redefined in the search process to be much closer. The search now continues until the closest possible value of x is found (within the limits of finite precision arithmetic in Excel). The resulting x should have a BETADIST(x, alpha, beta, A, B) value that differs from p by about 10^(-15).
ConclusionsMany inverse functions have been improved for Excel 2003 and for Excel 2007. Some have been improved for Excel only by continuing the search process to reach a higher level of refinement. This set of inverse functions includes the following functions: