There's been quite a bit of controversy about the number of undocumented immigrants in the US lately - for example, Ann Coulter claims that number is 30 million, whereas others claim it's about 11 million (readers of my blog are data-savvy, and would dig into the details of such claims, of course). It's difficult to get a definitive count of something that is by definition 'undocumented,' therefore I focus on something that is more easily quantifiable - the number of illegal aliens that have actually been apprehended.

I found the data on the US Customs & Border Patrol (CBP) website, in the form of a table, as shown in the partial screen-capture below:

This is very interesting quantitative data on the topic, but I found it a bit difficult to digest in their tabular form. There are just too many numbers to try to keep track of in my head, as I look for trends and such. Therefore I imported the data into SAS and created a bar chart - this really helped me see how the numbers have changed over time. I also used SAS to calculate the grand total (nearly 50 million) and annotate it onto the graph using a large font.

Now that I could 'see' the data, it made me wonder if the rises & falls over time might be correlated with other things/events? For example, I wondered if the US became more serious about apprehending illegal aliens when it was in a state of war? So I got a list of the start & end dates of the major wars the US has been in, and added them to the graph (by annotating bar areas behind the graph).

Next I tried recessions. Perhaps illegal aliens come to the US looking for jobs, and perhaps the number of jobs available is lower during recessions(?) Assuming that the number of apprehensions is proportional to the number of people coming into the country illegally, let's see what the data says...

I also wondered if different presidents (or political parties) might have different "get tough on illegal immigrants" policies, and whether that might be affecting the number of apprehensions. Here's that plot:

And one other thing I thought I should factor in was population. This data covers quite a long time span, and the population is quite higher now that it was in 1925, therefore I thought it would be interesting to calculate the number of illegal aliens apprehended per US population for each year. Here is that plot:

Ok - that's a lot of plots ... would anyone care to help me interpret them?!? What other ways would you plot this data, or combine it with other data, to look for other trends and further insight?  And while you ponder those questions, here's some background music ... Phil Collins' Illegal Alien :)