It's entirely unreasonable since both time periods are taken into account if you've read Siegel's work; as in any 30 year period in history since 1802 has not yielded basically any higher than ~7%.
I cannot find that in the article you linked.
Given the volitile nature of the market I find it hard to belive one cannot find
some 30 periods which exceed 7%. (Just as one can likely find some that are much lower).
It really shouldn't be too hard to cherry pick some periods with very good returns.
I also don't see anything unreasonable about excluding pre-1900's stock info. In fact I'd prefer including only years following the establishment of the SEC and the application of universal accounting standards etc.
9.4%, however it was derived at that anonymous blog, was in all likelihood nominal gains and factored nothing with regards to the reality of inflation. Given the supposed sterling quantitative analysis credentials of the author of the blog it seems highly unlikely he "forgot" about inflation, but it being a blog with no identifying information or sources of any kind, well, let's just say it's suspicious at best.
It's not an "annonymous" blog.
Information about the author is provided, such as:
Education:
BA, Business Administration.
University of Chicago Graduate School of Business.
MBA, Quantitative Analysis and Information Systems.
New York University Graduate School of Engineering:
PhD program (3 years, part-time) Quantitative Analysis.
Professional Experience:
Over 20 years of management experience at Fortune 50 companies, including one of the 30 companies in the Dow Jones Industrial Average (DJIA). Positions included --
Manager, Information Systems for a division, reporting directly to the chief financial officer.
Manager, Financial, Sales and Marketing Systems for the sector/group that generated approximately half of this DJIA company's business.
I'm not vouching for his accuracy etc. But it appears you may have not read all the site linked before dismissing it. The answers to your questions can be found there. He it appears he even makes his financial models available etc. I'm just not interested enough to bother, yet I still see no reason to dismiss his data out-of-hand. I noticed the 7% ROI your article cites is based on dividend reinvested in the stocks. While that's a reasonable approach it's reasonable to not do so. Small differences in the moedling will produce different results. There is no one to do it.
BTW: It's not clear, at least to me, if when he (blogger) says "the stock market" if he's referring to the DJIA. If he is referring to DJIA, then comparing your site to the other is apples to oranges. And there's nothing necessarily wrong about focusing on the DJIA to get an idea about how the market is performing; it's what everyone does.
Fern