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Old 02-15-2012
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Thanks for your contribution. I agree with most of what you wrote, and I like your links with the exception of the one from Nicholas Taleb (the author of The Black Swan), being a financier his agenda is too obvious. Taleb is evangelizing what he thinks are the fat tail properties of the financial markets; this looks more like a misapplication of the Gaussian distribution.

My wish is that you were more familiar with the math - this stuff is well vetted, understood, and accepted. Yours seems to be the only dissenting view in this study, but I appreciate that. It forces me to be more rigorous.

This is a model and I'm not intending to bestow on it any more power than it deserves. The average of independent random variables always follow the Normal (or Gaussian) distribution, the trick is in the application. In this application, because the standard deviation of the global population is unknown and given my sample size of 30 datapoints, I'm using the Students t-distribution. Students t says I have sufficient datapoints.

Statistics will never produce a precise number; instead, we get a range, a variance, and a confidence interval. It can't say 'who' will have the expected expense; only that 'someone' will. We can only work with the data we have while taking every precaution to ensure its legitimacy, accuracy, and consistency.

The study says that there's a 90% chance that an individual's expected expense will be in the range from $295 to $457 per month or $3500 to $5500 per year, which is very close to the initial intuitive opinion of $60K over a 5 to 10 year period (post number 22 of this thread:

One thing that resonates through is the final outcome. All of us who've owned these buses for any real amount of time can look at the numbers above and say, "yeah, that's about right".

This study offers one more brush stroke in the overall picture a buyer paints when searching for a bus. It says beware, you may afford the bus, but not the upkeep.

Last edited by davidmbrady; 02-19-2012 at 09:26 PM. Reason: Clarification