Again, to repeat what others have said. ROI has many factors (not all of which have to do with the magazine itself). A lot has to do w/ the advertisers product and ad campaign. This can statistically be estimated by niche and the magazine can handle the ad campaign to try to take the guess work out of it....but even so, its impossible to actually track real world customers UNLESS you can do one of the following:
1) Use a promo code on your product linking back to a 'real world' ad source
2) Use a special 1800# and/or URL for an ad campaign
3) Ask customers how they found you and get a high response (and assume that they answered accurately)
Unless they start making magazines with interactive LCD's in them (like penny on inspector gadget had)...I just dont see a truly accurate way of doing it.
