ADMIN: Re: Re: Freight car distribution


Mike Brock <brockm@...>
 

As I noted in my message of August 18, we have just about drained the useful
information out of the frt car distribution thread. I also asked in that
thread for opinions regarding the acceptable "error" in the predicting
capability of what I refer to as the Nelson/Gilbert theory. The point has
always been, to me, that, while the theory is logically interesting, how
does it perform?

Walter Clark writes:

"Tim's and Dave's analysis was only on a few trains on a couple of
lines (UP and Southern, if I read everything correctly), but the
analysis yields results that are more accurate the more
trains/lines/months/years you check."

Well, I don't have the data from the Southern train but I do have Tim's results from his 1947 UP data and, of course, I have the 1949 UP data...which I copied to Tim and he analyzed. The theory worked fairly well with the 1947 data...777 box cars...but the 1949 data...almost twice the size...1325 box cars...in Tim's words "blew it all to hell".

"What I (and I think Tim and
Dave)am saying is that once you get a large enough sample size (and
the point I was trying to make using the poker chips, surveys and
birthdays is that the necessary sample size is a lot smaller and need
not be nearly as wide-spread as most people think) you can make quite
accurate projections for a much larger universe."

That's fine. I would have no problems with the theory IF it were "proven". IOW, suppose we had data from say 30 RRs, each of about 35 trains. If the analyzer studied the data and noticed that the % of foreign box cars on a given RR matched the national % of the foreign box cars for these 1050 trains, I would think we had a good theory. I would think it would predict the number of foreign box cars on a given RR for a much longer period of time...say a yr. If we only had the data from 2 RRs to work from I would conclude that we MIGHT have a good theory. If, however, we had the data from 3 RRs to work from and the data from the largest did NOT support the theory, I would try to alter the theory to match the data or accept the theory at great risk. Unfortunately, this is the case when one applies the theory to the 1949 UP data. IMO. If what I am saying is incorrect, please let me know.

The thread will remain open for replies. I would also remind the members that this is a discussion within the scope of the group. It should also be conducted within the rules of the group...meaning in a civil manner.

Mike Brock
STMFC Owner

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