Careful with Caching |
August 7th, 2022 |
| tech |
(I've added the "wrong" overlay.)
This is clearly incorrect: there's no way Massachusetts has more prisons than colleges. (MA actually has the largest ratio of colleges to prisons in the US.) After putting a link to the original source in the Facebook discussion, however, we found something pretty weird: people on mobile were seeing the incorrect map, but people on desktop were seeing a corrected one:
It turns out that Facebook was appending a tracking parameter,
?fbclid=... on desktop, but not on mobile. Normally this
wouldn't do anything, because the site would ignore that in
determining what page to return, but this site is apparently
configured with a cache.
Many sites use caches to make it easier to serve pages. If you ask
them for a page they'll give it to you (which might require a lot of
work to generate) and then save a copy. Then when someone else asks
for the same page, they can return the saved copy instead of putting
in all that work to regenerate it. The site has a cached copy of
/usa-prison-v-college in its cache with outdated content,
but since ?fbclid=... is always followed by a new token
those requests will never be found in the cache, and they return the
current, corrected, page.
If you have a cache, what do you do when you change the page, like they did here? There are two main approaches:
Have a way to tell the cache the page has changed and it should forget its copy.
Always cache for a short time. Even just one minute can take a lot of load off a server that is getting thousands of requests for a hot page.
In this case they apparently didn't do either: we were running into this yesterday, and as of right now the site is still returning incorrect data.
(On the original question, comparing the number of colleges to the number of prisons is pretty silly: if one state runs large prisons and small colleges is that any better than a state that does the reverse? Comparing the number of people in prison vs college would make much more sense.)
My bigger complaint is with Facebook, appending that annoying parameter to every link, than with poorly configured web servers!
MA used to have more colleges, but they all became universities when they were renamed by marketing departments.
I seem to recall Hawaii imports prison capacity from other states for detention, and obviously Massachusetts exports a lot of education. If all states had to be self-sufficient I'm guessing Hawaii would flip colour here, holding prison and college size constant?
wait are they counting by institution and not headcount or capacity?
Insane, what we're being fed. Or conversely, very crafty.
At the bottom of the source page, it does have a graph of more like the thing you want: incarceration rate and degree rate (where MA is basically the best state on both, Maine might be a little lower on incarceration.) Ironically they only count bachelor's degrees and not associates' degrees, which seems kind of irrelevant given that 25% of people who have been incarcerated don't have a high school diploma or GED. If you care about reducing incarceration, better access to GED programs, vocational programs and associates' degrees are going to be much more useful than increasing the number of bachelor's degrees. https://www.prisonpolicy.org/reports/education.html
The “original source” you link to has numbers for MA of 88 colleges and 107 prisons, which matches the info in the infographic.
On a completely unrelated subject, the Globe had a piece this morning that said (among other things) that people with higher levels of education were more likely to make logical and mathematical errors than those with less education when the correct answer contradicted their political beliefs.
Marcus This summary is entirely backwards, if this is the research I think it is. They gave people data which on a quick skim appeared to support one position, but if you dig in you'll find it supports the opposite. When the quick skim contradict…
Paul I think this was a different study. They had a math problem with gun laws and crime data. The higher the education level the more likely liberals were to get an incorrect answer that the gun laws reduced crime and conservatives incorrectly…
Marcus perhaps just link the study you're summarizing?
Jeff Kaufman https://www.bostonglobe.com/.../us-vs-them-paradox.../ It didn't give enough information to link to the actual study unfortunately.
I think it's this one based on Googling though: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=2319992
Yep that is exactly the study I was talking about.
Paul Why do you think it was deceptive? The questions seemed quite clear to me. Furthermore, if the problem was people digging into the data more carefully you would see the same phenomenon with the apolitical skin rash question.
I wouldn't call it deceptive (nor did Paul). Here's how they describe it: "Correctly interpreting the data was expected to be difficult. Doing so requires assessing not just the absolute number of subjects who experienced positive outcomes (“rash bett…
Michael I don't understand how that disproves their point in anyway though. It still shows that people who are otherwise good at solving this sort of math problem, fail at doing so when confronted with data that contradicts their political belie…
Marcus Right, whereas people who are less good at it fail at solving it whether it contradicts their political beliefs or not.
I don’t understand this. The HTML document has a `cache-control: no-cache, no-store, must-revalidate` header, so why doesn’t the response contain the latest version? Where is the outdated document cached? Not in the browser because the browser fetched the document every time from the network. Then on Cloudflare’s edge servers? Why would Cloudflare cache document URLs with and without the query string separately?
Famously, the two hardest problems in computer science are cache invalidation and picking names for things.
I'm curious what's actually doing the caching here. Most modern servers and CDNs are fairly sophisticated about what components of the URL go into the cache keys, and know that tracking IDs should be ignored.
No, famously the two hardest problems are cache invalidation, naming things, and off-by-one errors.
You're saying that Dagon was off by one problem?