RE: [CTPP] Some more points

Bob, Well, The first block groups, in 1980, were definitely done by computer. They had no human input. After they were done, the bureau decided include long form data by block group on STF3, and the private sector loved it. (This was the replacement for Fifth Count File C, which for 1970 had provided data by enumeration district.) For 1990, most block groups stayed the same. Yes, it's true that the RO staff did the BNA splits (usually down main roads, which meant areas that were meaningless statistically), and we in the tract committees did have the opportunity to recommend block group lines. I just don't think that was done very often; it's hard enough to get people to do tracts. So, for the most part, the 1980 accidental geography remained in place. STF3 was expanded considerably so more data were available by block group. I have no sense that there was anything different this time than in 1990, except that you added that useless fourth digit and renumbered blocks within BG and tract. So, I stand by my original statement for areas which did not have good local intervention: block groups are accidental geography and not statistical geography. Re TAZs: yes, the purposes are very different. However, in the Detroit area and I'm sure in lots of other places, TAZs tend to follow tract lines in residential areas (where there is no high employment density). Of course the tracts are split up in downtowns and other high attractor areas. TAZs don't change very much either unless there are major land use changes that change their status as residential or attractor areas. As you know, I disagree with the fundamental point that the main objective in tracts is comparability over time. The main objective is tracts that work well now and into the future. Most of the time in most places, the changes are small enough that most tracts can and should be maintained from census to census, but comparability is not the main reason. The main reason is that the characteristics that make a good tract haven't changed. I know that you are quoting the official Census Bureau "line" about these matters. I am trying to help people understand what's really happening. Long form block group data are lousy. Someday, we're going to have another geographic conference where we can discuss these things. Meanwhile, you might want to look at a paper I wrote for PAA a couple of years ago. If you don't have a copy, I'll be happy to send it to you. ----------------------- In response to the writers from Spokane, Missouri, and RI: Those of you who have specifically delineated block groups are in that 5% (OK, maybe 10%) in the nation. I agree that people working in small metros and who are already concerned about their tracts are most likely to do the work. The more, the better! I'm all for it. However, it's still true that the vast majority of block groups are not specifically drawn as statistical geography. (As Bob LaMacchia mentioned, the bgs in the 1990 BNAs were drawn in the regional offices with an eye toward meeting field work needs; these needs are very different from those of data users!) The important thing for everyone to remember, though, is the essential shakiness of bg data. Your error rates are very high. Yes, the Detroit metro area is more than 10 times the size of Spokane, so it makes it easier to use tracts here. Congratulations to RI for merging/nesting census and TAZ geography, I think that's the best way to do it. Patty Becker -------------------------------------------------------