Re: [CTPP] Rounding inside the CTPP

The rounding within the CTPP data can play heck with doing any data analysis. In the Chicago Central Area there are 155 individual TAZs. If you take a simple table from Part 2, say mode to work by sex, some interesting things happen. If you sum the total workers using the "total" field you get 631,999. This becomes an important number because people like to know the total. However, when you sum all the modes by zone you get 631,883. This is not a big deal except if you want to show drive alone, carpool, transit and other with their modal share percents. In this region, some of us like to see the actual numbers along with the percents. Logic would say to use the 631,883 when calculating the percentages but then that means the sum of the totals (which we know to be the better number because row rounding was applied to the tables) 631,999 gets tossed aside. One could get creative and distribute the 116 workers in some weighted fashion which would not likely affect any percentages but then the next guy who comes along using the CTPP data and software would get different numbers and we are back splitting hairs over who got what number from where. Are others finding the issue of rounded numbers a bit frustrating, especially when it comes to aggregating TAZs? I suppose one way to deal with this would be to simply round everything to the nearest 100—Even the 1980 and 1990 data like it appears Chuck Purvis, MTC, has done with his Commuting to Downtown trend analysis. http://www.mtc.ca.gov/whats_happening/press_releases/rel263.htm -- Ed Christopher Planning Activities Resource Center Federal Highway Administration 19900 Governors Drive Olympia Fields, Illinois 60461 708-283-3534 (V) 708-574-8131 (cell) 708-283-3501 (F) _______________________________________________ ctpp-news mailing list ctpp-news(a)chrispy.net http://www.chrispy.net/mailman/listinfo/ctpp-news