[CTPP] CTPP issues

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