By historical data are you performing an actual vs predicted type of calculation? PE has a PrevEvent function that can access historical data...really depends on the type of calculations, and volume of calculations you need to perform when considering PE, ACE, ...
For example, let us assume you stored future data with a 1 day offset to a corresponding PI Point. Your comparison PE could be something like:
if 'ActualTag' = PrevEvent('PredictedTag','*-1d') then "WeGotItRight" else "WeGuessedWrong"
Check out PIPC\Help\PEReference.chm help file for all the functions available.
Another usefull PE Function to take a look in the documentation that Rhys recommended is TagVal(tagname [, time]).
@Keerthi: We have presented at the last vCampus Live! 2011 a hands-out session on how to apply future data in the PI System. If you missed it, you can download the material from the Download section of vCampus under the category Extras and the name is vCampus Live hands on sessions.
The hints you will find inside this document will let you use all analytical tools of the PI System (PI ACE, Performance Equation or PE, Totalizer, PI AF Formula, etc.) to compare actual and predicted values.
If you want to learn more on PI ACE, I suggest you take time to watch a previous Builders Café Webinar from the (PI ACE 2010 R2 in Action) on the topic which you can find here.
On PI Event Frames, I suggest this other Builders Café Webinar (PI Events Frames - Find your data by events) which you can find here.
The example provided by Rhys on PE is also valid for a simple comparison. I suggest utilizing YouTube as a fast way to learn Performance Equation by browsing the link here.