Montana’s American Community Survey is composed from the final interviews conducted with 10,138 households in the state. Since Montana has 519,935 households, the chance of any household being in the final interview is 10,138 out of 519,935 = 0.0195, right around 2% of the population is included in the survey. Since Tregp shows 295 households, we can guess that our community data is assembled from somewhere around 6 completed interviews.
This table about sample sizes is from kenpro.org
As you will note, a population of 250 calls for 152 samples, and that the break is in the lower right corner – while a sample of 382 covers a population of 75,000, 384 is good for a million. I’m not going into details about sampling – this is a blog, not a stats class. If enough people ask for the stats instruction, I’ll do another article. Suffice to say, we can expect the numbers on the ACS to be pretty vague in small communities. So let’s look at the ACS data: data.census.gov
The top of the page shows:
Total Population 515
Median Household Income $36,458
Bachelor’s Degree or Higher 26.1%
Employment Rate 40.2%
Total Housing Units 283
Without Health Care Coverage 9.0%
Total Households 249
As we go further down the page, we start to encounter the variance – the range the number represents. That 515 population is taken from the decennial census not the ACS.
Median Age: 60.5 +/- 3.2 years
16.8% of Trego folks speak a language other than English at home – plus or minus 15.4% so that’s somewhere between 6 and 166. Probably not a particularly useful piece of information.
That Median Household Income turns out to be plus or minus $8,980: the number can be as much as 24.6% off either way. It can be as low as $27,478 or as high as $45,438. As we move into the full chart on that number, we see that the number of households lists the margin of error as plus or minus 67. Could be as low as 182 or as high as 316.
That 26.1% of Trego residents with a bachelor’s degree or more has a 12% margin of error – it could be as low as 14.1% or as high as 38.1%, It shows 8.1% of our residents holding graduate or professional degrees, but doesn’t give a margin of error there.
The school board will be pleased to know that 97.8% (plus or minus 6.6%) of our kids are enrolled in Kindergarten to 12th grade. Might even surprise the County Superintendent. Pretty sure some kids out there are home-schooled.
That 40.2% employment rate (+/- 12.9%) looks low – but I guess it fits right in with a median age over 60 and 31.1% (+/-9.4%) disability.
And finally, there are 48 women 15 to 50 years old – but the margin of error is 38, so it translates to somewhere between 10 and 86.
The ACS data is good – but the sample for Trego was small, and not checking the limitations lets us make blunders.