What Does 20 People A Day Really Mean?

Warning: I play with data in the post below. My last post was fun pictures; this one might be a little dry.

I hear it all the time. I even repeat it to other people. I have no idea where it came from.

            "Every day, twenty people are moving to Durham!"

After hearing and saying those words for a while, I realized I didn't know what that actually meant. Were twenty people moving to the city? the county? the metropolitan region?

I wanted to get slightly more granular to understand exactly what this meant and WHERE in Durham they are moving to.

The Approach

I decided the best approach (i.e. the approach I had access to) was to look at data that the census puts out every year called ACS. Essentially, they survey a few households and use that to estimate a number of different measures including population.

I chose the most recent data published, 2017, and compared it to the previous year, 2016*. That shows how many more people live in Durham in 2017 than in 2016. All I had to do then was to divide by 365 and I had the daily numbers.

BUT WAIT! There is another way that population can fluctuate besides people moving to a region. People are born and people die. I guess you could call babies born into Durham as having recently moved here, but I went ahead and subtracted those numbers out to get migration alone. For those interested how, I will provide more detail at the end**.

Overall, I wanted to get a rough idea of migration to Durham. Now, I am curious if anyone out there knows where the "twenty people each day" number comes from and what that methodology is, because what I found was much different.

What I Found

Let's look at the migration per day as I found it. Every day:
  • 11.5 people move to the Durham-Chapel Hill Metropolitan Area (MSA)
  • 11.5 people move to Durham County
  • 10 people move to the City of Durham
Hmmm... that seems low. Now, these numbers are net (the number of people who move in minus the number of people who move away). Side note: this is also the reason that the larger MSA can have the same number as just the county, people moving out of nearby parts can offset migration in.

Maybe the 20-people-per-day number is talking ONLY about the gross migration to Durham. To me, that would seem misleading. For example, if you had 20 people moving to a city, but 25 people moving away, you would have a gross of 20 people moving here, but a net of -5. The effect would be much different from the implication that population is booming due to migration. Obviously, in this case, more people truly are moving to Durham, but maybe not as many as people say on net?

Another side note: for comparison's sake, I found that about 60 people per day are moving to the Raleigh Metropolitan Area (MSA), net.

Where Are They Moving?

I did go one step further, which was really interesting. I wanted to know WHERE people were moving within the city. ACS data can be broken down into census tracts, which are smaller parts of the county. I found that of the 11.5 people moving to Durham County every day:
  • 0.8 people move to downtown (more accurately, census tract 22)
  • -0.8 people move to the "Urban Tier" area*** (people are moving OUT faster than they are moving in!)
There were only two tracts where more than 2 people per day move, census tract 18.06, which is in the eastern part of the county and includes areas around Falls Lake and census tract 20.18, which is in the western part of the county, just south of 15-501.

Meanwhile, in census tract 10.01, which is part of East Durham, 1.65 people are moving OUT of that area. Again, I don't have an analysis here, but it I think overall, the breakdown of this data gives me a better understanding of migration to Durham. I may look at this effect again once 2018 data becomes available.

At this point, it is important for me to point out that it isn't necessarily true that two people a day are moving to an area close to Falls Lake year after year. This is JUST for the time period between the end of 2016 and the end of 2017. Maybe one gigantic development made a huge impact in that particular year, so don't take TOO much away from this, but it is still interesting.

What do you make of these numbers? Come let us know in the forum post here.


*I used the ACS 5-year averages, which may not mean anything to most people, but in case you are familiar with this data. It doesn't mean the populations are predictions for the average over 5 years, just that 5 years worth of data was used to make the prediction for 2016 and 2017.

**I used a great resource http://wonder.cdc.gov to find birth and death data for Durham County. From those numbers, I was able to calculate a net birth/death rate per 100,000 people for 2017 (about 672 or so). I applied that rate to the population of each of the areas that I looked at, which gave me the net births (births - deaths). I then simply removed that number from the population difference, so I was only looking at migration in or out of the area.

***I followed Durham's definition of the "urban tier" as closely as possible by choosing census tracts. In all cases, the borders don't line up, but they were as close as I could get them. Census tracts included were as follows:


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