Re: [CTPP] Working with County flow data

I see one reason why aggregation is causing larger, rather than smaller, standard errors. The formula used, known as Bienaymé's formula, assumes that each variable you are adding is independent of the others. In a map of the counties, this is clearly not the case, since we know the counties are autocorrelated. Perhaps you can estimate the covariance between counties and use that to more accurately estimate the variance using the formula Var(sum(X_i))=sum(sum(Cov(X_i,X_j)). This is the more appropriate way to calculate variance of a sum of random variables that are not independent. Isn't there a statistician in the room? Steve Steven Farber, Ph.D Assistant Professor Department of Geography University or Utah http://stevenfarber.wordpress.com -----Original Message----- From: ctpp-news-bounces(a)chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of liang.long(a)dot.gov Sent: March-12-13 10:18 AM To: ctpp-news(a)chrispy.net Subject: Re: [CTPP] Working with County flow data I can see why Census doesn't recommend do more than three variables at a time. When you add 17 counties together, you get a much bigger area with more households sampled. In theory, you should get a smaller MOEs compared each individual county. But if you derive MOEs from those 17 counties, you will get a much bigger MOEs, which is contradictory to the theory. ________________________________________ From: ctpp-news-bounces(a)chrispy.net [ctpp-news-bounces(a)chrispy.net] on behalf of Ed Christopher [edc(a)berwyned.com] Sent: Tuesday, March 12, 2013 11:15 AM To: ctpp-news(a)chrispy.net Subject: Re: [CTPP] Working with County flow data Thanks--I know the spread sheet allows you to recalculate MOEs for more than three variables but I remember doing more than 3 a while back and I was getting some wild MOEs. When I dug into it I found something in the Census compass reports that said not to do more than three variables at a time. I was hoping that someone figured out a way around this. Ed C On Mar 12, 2013, at 9:59 AM, "Hoctor Mulmat, Darlanne" <Darlanne.Mulmat@sandag.org<mailto:Darlanne.Mulmat@sandag.org>> wrote: The New York State Data Center developed a Statistical Calculations Menu that includes an option for computing the margin of error for the sum of three or more estimates. See attached. Darlanne Hoctor Mulmat Applied Research Division - Criminal Justice/Public Policy San Diego Association of Governments 619-699-7326 From: ctpp-news-bounces@chrispy.net<mailto:ctpp-news-bounces@chrispy.net> [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Ed.Christopher@dot.gov<mailto:Ed.Christopher@dot.gov> Sent: Tuesday, March 12, 2013 6:57 AM To: ctpp-news@chrispy.net<mailto:ctpp-news@chrispy.net> Subject: [CTPP] Working with County flow data Has anyone come up with some easy ways for collapsing and grouping counties together using last week's county flow data and recalculating new MOEs. I have so many counties that I want to group together that I am looking for a quick way that can handle "lots" of counties. Another issue I am struggling with is that we are always told not to group more than three variables at a time or the formulas for calculating the new MOE do not really work. This is particularly troublesome especially if I am trying to group 17 counties together. What it comes down to is 9 different calculations given that I can only group 3 counties at a time together. Anyone figure out any short cuts or ways around this short of disregarding the MOEs altogether? Given all the clustering that I am looking at using the "cheat" sheets I am used to, I will be recalculating MOEs for weeks. Ed Christopher <StatisticalCalculationsMenu.xls> _______________________________________________ ctpp-news mailing list ctpp-news@ryoko.chrispy.net<mailto:ctpp-news@ryoko.chrispy.net> http://ryoko.chrispy.net/mailman/listinfo/ctpp-news _______________________________________________ ctpp-news mailing list ctpp-news(a)ryoko.chrispy.net http://ryoko.chrispy.net/mailman/listinfo/ctpp-news