Explanatory Preamble
To understand the following discussion you first have to know all the jargon, so here are the explanations for the alphabets soup.
The Earned Value Technique, loosely referred to as "EV" is a project management control tool defined as: "An objective measure of the value of work performed expressed in terms of the budget planned for the work." In other words, what you got for what you spent. The key acronyms are:
EV 
= 
Earned Value = percent complete x corresponding budget 
BCWS
 = 
Budgeted Cost of Work Scheduled, now more properly known as
the Planned Value (PV) 
ACWP
 = 
Actual Cost of Work Performed, i.e. the actual cost 
BCWP
 = 
Budgeted Cost of Work Performed, now more properly known as
the Earned Value (EV) 
CV
 = 
Cost Variance = earned minus actual = BCWPACWP 
SV
 = 
Schedule Variance = earned minus budget = BCWPBCWS
BCWSACWP = Spending Variance = budget minus actual 
BAC
 = 
Budget At Completion 
EAC
 = 
Estimated (cost) At Completion
EAC = variance at completion 
CPI
 = 
Cost Performance Index = BCWP/ACWP 
SPI
 = 
Schedule Performance Index = BCWP/BCWS
Combined costschedule index = CPI x SPI 
You can extrapolate performance to date to calculate the EAC as follows:
EAC = BAC/CPI = BACxACWP/BCWP
This calculation assumes, of course, that the performance experienced to date will continue unchanged to the end of the project. For a variety of reasons it never is and so this calculation is in fact of no practical value.
You can see the relationships of these various elements by going to http://www.maxwideman.com/issacons3/iac1343/sld006.htm
