I also did comparisons of CTPP Part 1 vs. SF1 population for our region. On a locality basis, the differences were +/-2 persons, as I was told to expect. On a TAZ basis, the differences were greater. The differences ranged from +377 (3800 CTPP vs 3423 SF1, or +11%) to - 443 (210 CTPP vs 653 SF1, or -68%). A scatter plot of TAZ population from CTPP vs from SF1 resulted in an equation of [CTPP = 0.9998*SF1 + 0.6], with an R-square of 0.9983. I don't have a full understanding (as I guarantee few people do) of how the Census takes sampled data and converts that to estimates of the full population, and why that results in SF3 pop being different from SF1 pop. It would seem that they could be consistent, even at smaller geography. It would certainly make life easier for the users of the data. But given that they are not, this data (like all data) has its caveats, and so we just need to use the data appropriately. Andrew Pickard, P.E. Senior Transportation Engineer Hampton Roads Planning District Commission 723 Woodlake Drive Chesapeake, VA 23320 Phone: (757) 420-8300 Fax: (757) 523-4881 E-mail: apickard(a)hrpdc.org Web: www.hrpdc.org Hi all, Our MPO is currently checking the newly-released 2000 CTPP data. We have found some discrepancies between CTPP and other sources. According to Census, "CTPP Part 1 data will not agree with SF 1 or Redistricting (Full count data). The CTPP data are based on the census long form, and are subject to weighting, just like other sample surveys. The SF1 and Redistricting data are full counts, and are more accurate. The best source to do spot-checks on the CTPP data is SF3". Even keeping this in mind, we still found the comparision result alarming. For the population data, we aggregated census block population from the redistricting file into each TAZ, hoping the aggregation result is not significantly different from the CTPP data. We have 602 TAZs in our area, of which 511 have somebody living in them according to the redistricting file. However, according to CTPP, only 474 TAZ have population. The population in the 37 zones that CTPP ommited varies from 1 to 40, which we were told are not significant. We do have some zones that have significant differences in population (at least we consider "significant"). We have a zone that has a population of 1277 in the redistricting file. The CTPP says that this zone has a population of 455. The difference between these two data sources are 822. We are not sure how to deal with this problem. Do any of the other MPOs have the same kind of problems? We know we need to use the CTPP information with care, but we assume that no matter what sampling method and what weighting factors the Census is using, a difference of almost 200% is still too much. We would like to pose this question to all of the CTPP users. If any of you can give us a good explanation or how your agency is addressing the data issue, please drop me a note so that we all learn from it. Thanks. Sincerely Zhongze (Wykoda) Wang Associate Transportation Planner Chatham County - Savannah Metropolitan Planning Commission P. O. Box 8246, 110 East State Street Savannah, GA 31412-8246 Phone: (912) - 651-1452 Fax: (912) - 651- 1480 Email: wangz(a)thempc.org