The Binomial Distribution is a statistical measure that is frequently used to indicate the probability of a specific number of successes occurring from a specific number of independent trials.
For the Negative Binomial Distribution, the number of successes is fixed and the number of trials varies. The function calculates the probability of a given number of failures occurring, before a fixed number of successes.
The following two forms are used:
Given the probability of a success from a single event, the Excel NEGBINOM.DIST function calculates the probability mass function or the cumulative distribution function for the Negative Binomial Distribution.
This gives the probability that there will be a given number of failures before a required number of successes is achieved.
For example, given the the probability of 50%, that the toss of a coin will land on heads, the Negbinom.Dist function will calculate the probability that you will toss 10 tails before you toss 12 heads.
The Negbinomdist.Dist function is new in Excel 2010, and so is not available in earlier versions of Excel. However the negative binomial probability mass function can be calculated by the Negbinomdist function, which is available in earlier versions of Excel.
The syntax of the Excel Negbinom.Dist function is:
where the arguments are as follows:
number_f    The number of failures encountered before number_s successes.  
number_s    The required number of successes.  
probability_s    The probability of success in one trial.  
cumulative    A logical argument that specifies the type of Negative Binomial Distribution to be calculated. This can be either:

Note that, if the number_f and number_s arguments are input as decimals, these values are truncated to integers.
In the above chart on the right, the Negative Binomial Probability Mass Function is used to show the probability that a tossed coin will result in exactly x tails before there are 12 tossed heads.
The spreadsheets below show the Excel Negbinom.Dist function used to evaluate this function for three different values of x.
Clearly, the probability of tossing a head on any one trial is 0.5, so this is provided as the value of the probability_s argument.
Formulas:
 Results:

In the above chart on the right, the Cumulative Negative Binomial Function is used to show the probability, that a tossed coin will result in at most x tails before 12 tossed heads.
The spreadsheets below show the Excel Negbinom.Dist function used to evaluate this function for three different values of x.
Formulas:
 Results:

Further examples of the Excel Negbinom.Dist function can be found on the Microsoft Office website.
If you get an error from the Excel Negbinom.Dist Function, this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if either:
