Mara--Let me know if you hear anything.  I think this is a very perplexing question and worthy of some research under the banner of "how to work with ACS data, tips and tricks".

Attached is something I did that might give you "a little" insight.  On the attached slide I had a block group table with 5 different modes to work, the Total Population and Total Workers.  What I wanted to do was to figure out the total number of commuters along with the MOE. Remember Census does not give us the Total Commuters with the block group data. Doing the math to get Total Commuters was easy, just add up the modes. However, the MOE became something of a guessing game.  I first used the formula for for calculating the MOE for more than two estimates.  I got one number 214 (with 4 variables) or 245 (with 5 variables) depending if you used the MOE associated with a cell that had zero observations. Dealing with zero cells in ACS brings its own issues and questions.

I then realized that the total commuters was actually the difference between the Total Workers and those who Worked at Home, only two variables.  That allowed me to use a different MOE formula (which by the way is the same even though I was subtracting values instead of adding) and I got 209.

My point is that although small with only 4 variables I was introducing some additional error by increasing the number of variables.  Exactly how much I wasn't sure. If I did not consider the zero cell MOE influence, the added error was in the neighborhood of 2.3%.  So what I found is that as you increase variables your MOE calculation would then have an additional MOE.  Hey a new statistic, the MOE of the MOE.

What I could have done is a few more Block Groups and found some with no zero values so that I could see what 5 variables as opposed to 4 did but that is where I think this is worthy of research by someone with a statical mind.

mkaminowitz@baltometro.org wrote:

Hello,
I need to get a regional estimate and MOE composed of 6 counties. I know the rule of thumb is that aggregating MOE's doesn't work well for more than 4 units. Does anyone know is using 6 would cause the MOE to be inaccurate?
 
*Mara Kaminowitz
*GIS Analyst
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