Return on Investment for Advertising

Return on investment is calculated using the equation
                                                              (Return - Investment)  X 100%
                                                                        Investment
or,
                                                 Change in Profit due to Campaign   X 100% 
                                                            Cost of the Campaign

In marketing this can be quite difficult because you have to determine what your return is from the particular marketing campaign. This is often done by asking customers how they heard about you, but isn't always accurate.   

For example if you sold 100 widgets at 1 dollar each and 25 customers answered they heard from your news paper ad then you can figure out your RoI. If the add cost you 20 dollars then:
                                                             (25 - 20)  
                                                                   20         equals .25 so 25% is their investment.
It is hard to figure out how or even if your advertisements are working, but some ways are customer questioners, and or surveys. Possible incentives for answering never hurt. (unless your RoI is in the negative them you may have something to lose)