Washington County was 997th out of 3005 counties in the US in terms of per capita income, 55th out of the 99 counties of Iowa, for 2009 returns
The IRS provides county level data on tax returns for the whole US.
Using the free stats program R, we can easily play around with the numbers a bit. For instance, where does Washington County rank in terms of per capita income relative to the rest of Iowa and to the country as a whole?
Looking at the 2009 data (the latest available):
> d <- read.csv("Downloads/09incicsv.csv", stringsAsFactors=FALSE) > names(d)  "State_Code" "County_Code" "State_Abbrv" "County_Name" "Return_Num" "Exmpt_Num" "AGI"  "Wages_Salaries" "Dividends" "Interest" > sum(d$State_Abbrv == "IA")  100
Poking around the file, we find it contains info on some non-counties too. (Eg, we find 100 rows for Iowa, not just 99.) The counties, sensibly enough though, seem to have have "County" at the end of their names. Select just those:
> counties <- d[grepl("County$", d$County_Name), ] > sum(counties$State_Abbrv == "IA")  99
One last wrinkle is that to avoid revealing personal info, the numbers for counties with very small numbers of people are suppressed, replaced with -1. Let's filter those out:
> nrow(counties)  3006 > counties <- subset(counties, AGI != -1) > nrow(counties)  3005
The rank function orders items from smallest to largest. So, to get the largest first, just multiply by -1:
> counties$rank <- rank(-counties$AGI / counties$Return_Num) > subset(counties, State_Abbrv == "IA" & County_Name == "Washington County")$rank  997
That is, roughly, 1/3 of counties in the US are richer per capita than Washington County, looking at adjusted gross income.
> ia <- subset(counties, State_Abbrv == "IA") > ia$rank <- rank(-ia$AGI / ia$Return_Num) > subset(ia, County_Name == "Washington County")$rank  55
So, more than half of Iowa counties have per capita income higher than Washington County.
Do these numbers look right to you?