MDX Query percentile 25th, 50th and 75th - mdx

Question

I have a question and I haven't been able to find the answer (neither in this forum nor other) I am looking for:
I need to calculate the 25th Percentile, the median (the 50th percentile) and the 75th percentile.
Putting in another words: I need to write in the MDX query in SSRS for it to tell me which data is the 25th, the median and the 75th
All I was able to find so far was not the exact values of each one of them
thanks

Answer 1

I've been working on the same issue for my own data. The trouble I was having is in figuring out the Median() function. Here's how I interpret the parameters of the function:
Microsoft's definition:
MEDIAN(Set_Expression [, Numeric_Expression])
My interpretation:
Set_Expression is the set of values that define the grain to which the measure is summed before the median is evaluated
Numeric_Expression is the measure that is summed, which set of sums is then sorted and evaluated to find the median
In my case for finding the straight median across the entire data set, I didn't want to sum the values at all. To prevent any sums from being calculated, I used the key attribute for a dimension that had a 1-1 cardinality with the records in the fact table that contains the measure that I'm using. The only flaw I've seen so far is that sometimes the median returns a whole number when there are an even number of records and the mean of the two middle records should result in a number ending in .5. For example, the values of the two middle records are 16 and 17 and the function is returning 17 instead of 16.5. Since this is a minor flaw, I'm willing to overlook it for now.
This is what my calculation with the median function looks like:
WITH MEMBER Measures.[Set Median] AS MEDIAN(
[Dimension].[Key Attribute].MEMBERS
,Measures.[Non-summable Measure]
)
I used a combination of Median and TopCount to get the 75th percentile. I use TopCount to limit the set for the median to the second half of the data since TopCount sorts the data in descending order. I'll explain how I understand TopCount:
Microsoft's definition:
TopCount(Set_Expression, Count [, Numeric_Expression])
My interpretation:
Set_Expression is the set of values from which the desired number of tuples will be returned
Count is the number of tuples to return from the set
Numeric_Expression is the value that will be used to sort the set in descending order
I want the Median function to use the last half of the records in the fact table that are returned in the query, so I again use the key for the dimension table that has a 1-1 cardinality with the fact table and I sort it by the measure from which I want to find the median value.
Here is how I coded the member:
MEMBER Measures.[75th Percentile] AS MEDIAN(
TOPCOUNT(
[Dimension].[Key Attribute].MEMBERS
,Measures.[Fact Table Record Count] / 2
,Measures.[Non-summable Measure]
)
,Measures.[Non-summable Measure]
)
So far, this combination of functions has returned a true 75th percentile from my data set. To get the 25th percentile, I tried replacing TOPCOUNT in my code with BOTTOMCOUNT, which is supposed to do the same thing, only sorting the data in ascending order to use the first half of the records instead of the second half. Unfortunately, I haven't been able to get anything but NULL from this combination of functions, so I'm open to suggestions on how to get the 25th percentile.
This is how my final query looks:
SELECT
{
Measures.[Set Median]
,Measures.[25th Percentile]
,Measures.[75th Percentile]
} ON 0
,[Dimensional row members here] ON 1
FROM [Cube]
WHERE
[Non-axis dimensional filter members here]