There's a discussion over at TheForce.net asking what the sales were for Star Wars comics across time; it led into a discussion of Statements of Ownership, whcich I know a little about. Covering here what I said there:
The Statement of Ownership, found in U.S. Code as Section 4369, Title 39, has been required of publishers who ship Second Class since the 19th Century. But it was only in 1960 that it became the Statement of Ownership and Circulation, as that was the year that publishers were first required to list their average paid circulation for the year. That is sold copies, distinct from print runs; while the Postal Service did require publishers then to publish their print runs as well, some, like Dell, didn't bother until a couple of years later. The USPS also required publishers to say how many copies were sold on average by subscription, which was the whole point of adding circulation to Section 4369. Since the USPS was cutting publishers a special rate by letting them ship Second Class, they wanted them to prove that their publications were actually requested, and not junk mail.
In later years, the USPS would require many other statistics, including what the sales were for the most recent issue of the year in addition to the average issue. Today, Second Class is known as Periodical Class, and among major comics publishers only Marvel and Archie continue to use it; DC stopped in 1988. Wizard and most print magazine publishers continue to use it. I have filled out many of the forms myself in my magazine career.
Historians using Statements of Ownership can compute things like the number of copies returned from dealers -- essentially, unsold copies which were destroyed -- and the percentage of the print run that was sold in any given year. The Statements are inspecific -- they give an average of a year rather than data for all the specific issues -- but it is some of the only hard information available for the 1960s and 1970s. Data from the 2,500 or so Statements of Ownership I have found appear in my aforementioned book, the Standard Catalog of Comic Books.
The reliability of the Statements in the 1970s and onward is generally pretty good, given the comparisons I've been able to make with archival info from the publishers themselves. Looking at them across an entire year or range of years for a publisher, it's pretty easy to see when a figure is outside what we'd expect. There are more simple math errors than there are attempts at distortion, I find. Again, nobody was looking at these things. Statements of Ownership were generally not used by publishers for advertising purposes -- they were simply an obscure postal obligation, and far more of interest to the fans who actually saw and read them. Advertising rates for Atlas/Marvel and National/DC were more likely to have been based on the reports of the Audit Bureau of Circulation, to which both belonged. (That's where the REAL data is -- locked up in the microfiche of the ABC's Chicago offices. I've sent a correspondent down there a few times to do some archaeology for my previous books.)
I cite all statistics from the Statements in my book, but in all the sales charts we only use the "average paid circulation" figure, and not the print run, which is higher.
Sadly, there is not Statement data for Star Wars before 1979; publishers did not usually have a second class license (or sell subscriptions) in the first two years of a title's run. I can say, however, that Star Wars #1-3 were the best selling comic books of the entire 1970s, period. There were two variant versions of the first printing of #1 and three for #2-3 -- and while they didn't add up to many copies -- perhaps 200,000 -- the two versions of Whitman bagged reprints that followed boosted the number of copies of those issues over 1 million each. They were the ONLY comics in the 1970s to top that mark. Star Wars would be the top selling title in the business for three more years -- and as former editor-in-chief Jim Shooter once said, Star Wars absolutely saved Marvel, which was close to being liquidated by its owners. Comics nearly died altogether in the mid-1970s, so you might say George Lucas helped save comics!
As for absolute copies in print of the same story, it's very difficult to say. I would imagine things like the movie adaptations by Dark Horse have appeared in many, many different printings and formats around the world and in many languages. If it is calculable, I'd sure like to know. I can add it to my trivia wall!