The k'th percentile of a range of data is the value that k% of the data values fall within.
This is explained in more detail on the Wikipedia Percentile pageIn Excel 2010, the new PERCENTILE.INC and PERCENTILE.EXC functions both find the k'th percentile of a supplied data set.
The difference between these two functions is that, in the Percentile.Inc function the value of k is is within the range 0 to 1 inclusive, and in the Percentile.Exc function, the value of k is within the range 0 to 1 exclusive.
The Excel PERCENTILE.INC function returns the k'th percentile of a supplied range of values for a given value of k, within the range 0 to 1 (inclusive).
The function is new in Excel 2010 and so is not available in earlier versions of Excel. However, Percentile.Inc is simply a renamed version of the old Percentile function, that is available in earlier versions of Excel.
The format of the Percentile.Inc function is:
Where the function arguments are:
array    The range of data values for which you want to calculate the k'th percentile 
k    The value, between 0 and 1 (inclusive), of the required percentile 
If k is not a multiple of 1/(n1), (where n is the number of values in the supplied array), the function interpolates between the values in the supplied array, to calculate the percentile value.
The spreadsheets below show examples of the Excel Percentile.Inc function used to calculate the k'th percentile of a set of values in cells A1  A6 of the sample spreadsheet, for different values of k.
The formulas for the functions are shown in the spreadsheet on the left, and the results are shown in the spreadsheet on the right.
Formulas:

Results:

Note that in the above examples:
For further examples of the Excel Percentile.Inc function, see the Microsoft Office website
If you get an error from the Excel Percentile.Inc function this is likely to be one of the following:
#NUM!   
Occurs if either:


#VALUE!    Occurs if the supplied value of k is nonnumeric 