The Cumulative Binomial Distribution (calculated by the Excel Binomdist function) is a statistical measure that is frequently used to indicate the probability of obtaining a specific number of successes from a specific number of independent trials.
The Critbinom function calculates the inverse of the Cumulative Binomial Distribution.
I.e. for a given number of independent trials, the Critbinom function returns the smallest value of x (the number of successes) for a specified Cumulative Binomial Distribution probability.
Further information on the Binomial Distribution can be found on the Wikipedia Binomial Distribution page
In Excel 2010, the Critbinom function has been replaced by the Binom.Inv function, which has improved accuracy.
Although it has been replaced, the Critbinom function is still available in Excel 2010 (stored in the list of compatibility functions), to allow compatibility with earlier versions of Excel.
The Excel CRITBINOM function returns the inverse of the Cumulative Binomial Distribution. I.e. for a specific number of independent trials, the function returns the smallest value (number of successes) for which the cumulative binomial distribution is greater than or equal to a specified value.
For example, the Critbinom function could be used to find the smallest number of tosses of a coin for which there is a 50% chance of at least 20 heads.
The format of the function is:
where the arguments are:
trials    The number of independent trials that are to be done. (If this value is a decimal, it is truncated to an integer by Excel). 
probability_s    The probability of success in one trial. 
alpha    The criterion value of the Cumulative Binomial Distribution (must be between 0 and 1). 
The above chart on the right shows the Cumulative Binomial Distribution Function for 100 tosses of a coin. This curve represents the probability that at most x heads will be thrown from the 100 tosses.
The Excel Critbinom function calculates the inverse of the curve on the right. I.e. the Critbinom function calculates the minimum value of x for which the Cumulative Binomial Distribution function is a specified value. This is shown in the spreadsheets below, for three different criterion values.
Formulas:
 Results:

Note that the probability_s and the alpha arguments can be entered as decimal values or as percentages.
Further examples of the Excel Critbinom function can be found on the Microsoft Office website.
If you get an error from the Excel Critbinom Function, this is likely to be one of the following:
#NUM!    Occurs if either:

#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 