I do lots of analysis with percentiles - and it's unfortunately harder than I'd like, but it is doable.
I wrote up a long description of the options I could find a while ago here (everything I said for percentiles applies to quartiles too):
At that time the answer was if you are using Oracle it's easy, otherwise it's really hard. I did manage to get an example going with SQL Server using ntile() (or similar) - but it was ugly.
With version 6 you can calculate the values in Tableau with Table Calculations. I posted the details of how to do that for inclusion in the Tableau Calculation Reference Library that James is working on.
It's also worth noting that Tableau directly supports median() as an aggregate function if you are using Oracle or the version 6 extracts with the Tableau Data Engine. You could add your voice to the lobby group for that to be extended to quantiles in general for databases that support it (Oracle and the Data Engine).
Thank you! This is very comprehensive and I really appreciate the help.
I'm using MSSQL, so I've used NTILE() like I mentioned, but I've not been able to figure out how to get beyond that. Extracts are probably not doable either simply because of how much data I have to work with. Thanks again for all your help.
*Edit* In other news, I used your tableau file from that link and tried to replicate your results for a good hour and couldn't... my graphs were always wrong. Then, I realized that the axes were wrong! Everything works great now and I've moved it over to my live dataset. Thanks again. */Edit*
Glad it was useful.
The NTILE() approach is really tricky. The way to achieve percentiles with NTILE() in SQLSERVER is explained in a link on some SQLSERVER site that I linked to in the first of my postings above. But as I mentioned, the full solution given there didn't work from Tableau because it used CTEs, which don't work from a custom SQL connection. I did manage to get some horrible intermediate kludge going, but unless your data volumes are absolutely enormous I think the Table Calc approach is much better.