I’m in a
statisticians dream, but my nightmare…..
Sonny Sanders, GISP
Capital Area Metropolitan Planning
Organization
City of
573-634-6439
www.jeffcitymo.org/campo
From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Jason Gillow
Sent: Monday, January 03, 2011
3:28 PM
To: ctpp-news@chrispy.net
Subject: [CTPP] RE: How to Combine
Median Values for Multiple Areas
Hello
I had a similar
issue median and average values, I had to contact the Census Bureau themselves
to get an answer.
Below was my
question and their answer to me.
I
have attached a revised version of you table with what I think is the
appropriate approximations of the MOE of means, medians and ratios. You
should check to make sure I found them all.
(See
attached file:
Please
note:
1.
I have changed the concept of your medians. They are now the average
median instead of the median of medians. I did this because it makes it
easier to calculate the MOE. We get back to taking the square root of the
sum of MOE^2 which is as given in the appendix you mention below. Except
now we divide it by the squared number of estimates in the average. In
this case 144 (or 12^2). The benefit of the shift is the MOE is much
smaller than what you had initially. I have to warn you that it may be an
underestimate since we have not adjusted for correlation between the estimates.
2.
I noticed that the median values didn't change much with the shift and the new
medians seem more in line with the distributions shown that are associated with
the medians.
3.
Your idea of taking the square root of the median MOE^2 is inventive,
but
all you did was assign an MOE from one of the other median estimates to
selected median of the medians.
4.
If you are unhappy with the concept change, you can go back to taking the
median of medians, but I am suspicious that it isn't the truly overall
median.
As far as the median's MOE, it would be a lot of work using
linear
interpolation and you probably won't get anything useful. Instead
just
use the median's own MOE and not the median of the MOEs. This result does
give an extremely conservative MOE so technically you cannot go wrong except it
probably isn't useful.
5.
Your average of the means concept was fine, but I made the same change to the
MOE formula as I did for the medians.
6.
I did check some of your count estimates and they seem to be calculated
correctly.
Again
I want you to understand that what you and I are calculating here are
approximations. They maybe overestimates or underestimates. But I feel
they are better than what you had. Hopefully they will be useful.
Karen
E. King
Supervisory
Mathematical Statistician
ACS
Variance Estimation and Statistical Support Branch Decennial Statistical
Studies Division
Phone:
301 763-1974
From:
Jason G
Date: 12/30/2010 09:24
AM
Subject:
RE: Regarding question about summing
estimates
Hello
Both
questions refer to the same matter. I work for a regional planning commission
that serves 12 counties in south central
-
profile tables. I am using the formulas referenced in http://www.census.gov/acs/www/Downloads/handbooks/ACSResearch.pdf
in Appendix 3 on page A-14 under section “Calculating MOEs for Aggregated Count
Data” to get the aggregate estimate and Margin of error (MOE) numbers for our
region. I know this works with like "total household" category for
example to get the aggregate estimate and its MOE. But the problem I have is
calculating the aggregate estimate and its MOE for "Average household size",
“median family income” and "Rental vacancy rate" for example. I have
figured out the formulas for the aggregate estimates of these types, but
I have trouble with the calculating the MOEs for these aggregate estimates for
like "Average household size", “median family income” and
"Rental vacancy rate" for example for the region.
Attached
is my main work table in Excel format for reference. The numbers in bolded red
color are for the region I serve, and gray highlighted cells are the MOEs I had
trouble with.
What
I need to know is the formulas for calculating the MOEs in the gray highlighted
cells in my spreadsheet.
Jason
Gillow
Research/Planning Specialist II
Ohio
Valley Regional Development Commission
9329 St Rt 220 E, Suite A
Waverly, OH 45690
Phone: 740-947-2853 or 800-223-7491 (In
Ohio)
Fax: 740-947-3468
www.ovrdc.org
Email: jgillow@ovrdc.org
From:
ctpp-news-bounces@chrispy.net [mailto:ctpp-news-bounces@chrispy.net] On Behalf Of Cook, Cliff
Sent: Monday, January 03, 2011
1:24 PM
To: ctpp-news@chrispy.net
Subject: [CTPP] How to Combine
Median Values for Mutliple Areas
I
am looking for guidance on how to combine median income values for several
census tracts into a single value and associated margin of error. I
recall seeing something about this topic in an ACS document but cannot locate
the information at this time. Any help would be appreciated.
Cliff
Cook
-----------------------------------------------------------------------------
Clifford Cook
Planning Information Manager
344 Broadway
617/349-4656 FAX 617/349-4669 TTY
617/349-4621
email => ccook@cambridgema.gov
web site => <http://www.cambridgema.gov/~CDD/>
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