Explanation:
The crime rate statistic shows the number of registered crimes divided by the number of registered residents in a country or area.
"Registered" is important here!

If you add undocumented immigrants to the calculation, the statistic is skewed:
Undocumented immigrants aren't registered as residents for the statistic, since they are, well, undocumented.
However, any crime committed by any one of them will count towards the crime rate when they're registered by the police.

So even if they were much less likely to commit crime than the resident population on average, the crime rate statistic would still increase. The denominator of the equation doesn't increase by definition, because only legal residents are counted towards the statistic. But the real number of people inside the country who may commit crimes increases.

This is important to know as context when people try to "prove" immigrants are more criminal than citizens, using the crime rate statistic.

Using your logic, I could drive to the next town over commit some crimes and have the same effect. Pinning it to undocumented people is a way of trying to frame the discussion in a xenophic way.

No, it's trying to give people ammo to counter when others frame the statistic in a xenophobic way.
You're right, the effect is more general, it leads to the Vatican having the highest crime rate in the world by far (<800 residents, millions of visitors).
But you don't often hear right wing agitators try to claim that tourism is bad.

Counter point, Superman. Illegal immigrant, made the crime rate drop overall by making it unsafe to be a criminal.

/j

Its a fair point, and well explained. However, I think it implies that illegal immigrants are fine so long as they're not more criminal than the general population.

Generally if somebody is using crime statistics to argue for more immigration control, they're probably the kind of person that believes the only acceptable amount of crime for an illegal immigrant to commit is zero crime.

I wouldn't call this so much a statistical error but a human one. While the exact number may be unknown, somebody with a pencil sat down somewhere and counted some beans on a sample, which a math whiz can extrapolate to get to a total population number that's closer to the truth.

Never trust a statistic, which you haven't forged yourself.

You could probably, for most regions in the world, make the reasonable assumption that, on average, the effect balances out; you might be able to drive somewhere and commit a crime, but so can someone else from somewhere else, and so on. Obviously, places which receive a lot of travel are exceptions to this rule, just like places where would-be criminal travelers reside (I'm looking at you, suburbia).

Of course, I'm just speculating in hopes someone else has some real data to back up my claims...

In the US our census counts residents, not citizens. While undocumented immigrants are historically under counted, this still seems to be based on the assumption that most countries don't include undocumented immigrants in population counts which is a flawed assumption.

Please provide some source that shows this is more than a shower thought.

Yes but they're victims of crime too. So even if the absolute number of crimes goes up with immigrants, citizens will each experience less crime.

Um Ackshually, there are many places where even "undocumented" people can be considered "registered" in this sense. "Undocumented" simply means lacking the authorization to be (and work) in the country, and is really just a minor paperwork violation unless an actual crime is committed in rhe process.

So, many localities will try and integrate these people into the community, and provide them the same local services everyone else gets, by simply not asking them to show their passport first. So, they might be able to get their name on utility bills, and use that as proof of residency to get a drivers' license, in spite of their lack of status. (This notion that undocumented people still deserve human dignity really irks Conservatives, even though they all had ancestors who held the same status here at some point.)

This is really at the heart of what a "sanctuary city" policy is all about. It's a recognition that a local government has a responsibility to all people living in it, regardless of their immigration status. And that local officials really don't have the tools to be able to determine that anyway, so a local official won't care about someone's immigration status unless they are on the wrong side if the law. (Which also exposes the lie that these cities are less safe. If anything, sanctuary policies make it easier to kick out the miscreants, because no one is spending time chasing down undocumented people who are not causing trouble, leaving more time to find the bad eggs)

This does depend on the methodology stated by the institution collating the data. The most recent methodological statement I can find from the FBI's stats reporting indicates it uses locality population estimates derived from the federal census - which in turn aim to count not the people each locality has registered as a resident, but the number of people who usually live and sleep in a locality regardless of documentation status.
Obviously, undocumented migrants may consciously attempt to be undercounted in that divisor but they don't necessarily disappear entirely.

The other angle on this I've seen multiple times is comparisons of crime rate per capita for populations segmented by migrant status like this, where the usual conclusion is "A person is less likely to commit crime, given that they are a migrant". It seems pretty believable to me that this reflects a raised background level of caution around crime caused by migrant status.

In the US our census counts residents, not citizens.

No. It counts residents who cooperate with the census. Guess what undocumented immigrants won't be doing?

Going door to door to count people is a terrible method of kering track of people, which is why most places stopped doing that, using running registries instead. It's pretty stunning that the US only has the vaguest concept of who lives there.

Using your logic, I could drive to the next town over commit some crimes and have the same effect.

Correct. It's pretty well known that this skew per capita statics in small-scale analyses. It's also why you shouldn't do per-capita analysis on very local scales

Then it sure is a good thing that the vast majority of the US census is not the result of door to door enumeration. Most people complete the census online or by mail.

arnt red states also using undocumented and prisoners as part of the census in thier states too,.

I think he's technically a legal immigrant. The Kents adopted him with paperwork and everything.

Forged paperwork though, at least in Smallville universe, so not completely legal.