NEGBINOMDIST function
Returns the negative binomial distribution. NEGBINOMDIST returns the probability that there will be number_f failures before the number_sth success, when the constant probability of a success is probability_s. This function is similar to the binomial distribution, except that the number of successes is fixed, and the number of trials is variable. Like the binomial, trials are assumed to be independent.
For example, you need to find 10 people with excellent reflexes, and you know the probability that a candidate has these qualifications is 0.3. NEGBINOMDIST calculates the probability that you will interview a certain number of unqualified candidates before finding all 10 qualified candidates.
Important This function has been replaced with one or more new functions that may provide improved accuracy and whose names better reflect their usage. This function is still available for compatibility with earlier versions of Excel. However, if backward compatibility is not required, you should consider using the new functions from now on, because they more accurately describe their functionality.
For more information about the new function, see NEGBINOM.DIST function.
Syntax
NEGBINOMDIST(number_f,number_s,probability_s)
The NEGBINOMDIST function syntax has the following arguments:

Number_f Required. The number of failures.

Number_s Required. The threshold number of successes.

Probability_s Required. The probability of a success.
Remarks

Number_f and number_s are truncated to integers.

If any argument is nonnumeric, NEGBINOMDIST returns the #VALUE! error value.

If probability_s < 0 or if probability > 1, NEGBINOMDIST returns the #NUM! error value.

If number_f < 0 or number_s < 1, NEGBINOMDIST returns the #NUM! error value.

The equation for the negative binomial distribution is:
where:
x is number_f, r is number_s, and p is probability_s.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article.
Important Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In Excel, create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V.
Important For the example to work properly, you must paste it into cell A1 of the worksheet.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

