Crémieux Profile picture
Oct 4 • 10 tweets • 4 min read • Read on X
There are people who desperately want this to be untrue🧵

One example of this came up earlier this year, when a "Professor of Public Policy and Governance" accused other people of being ignorant about SAT scores because, he alleged, high schools predicted college grades better.Image
The thread in question was, ironically, full of irrelevant points that seemed intended to mislead, accompanied by very obvious statistical errors.

For example, one post in it received a Community Note for conditioning on a collider. Image
But let's ignore the obvious things. I want to focus on this one: the idea that high schools explain more of student achievement than SATs

The evidence for this? The increase in R^2 going from a model without to a model with high school fixed effects

This interpretation is bad. Image
The R^2 of the overall model did not increase because high schools are more important determinants of student achievement. This result cannot be interpreted to mean that your zip code is more important than your gumption and effort in school.

If we open the report, we see this:Image
Students from elite high schools and from disadvantaged ones receive similar results when it comes to SATs predicting achievement. If high schools really explained a lot, this wouldn't be the case.

What we're seeing is a case where R^2 was misinterpreted.
The reason the model R^2 blew up was because there's a fixed effect for every high school mentioned in this national-level dataset

That means that all the little differences between high schools are controlled—a lot of variation!—so the model is overfit, explaining the high R^2
This professor should've known better for many reasons.

For example, we know there's more variation between classrooms than between school districts when it comes to student achievement.

As another example, we know that achievement gaps exist along the whole continuum of school and district quality.

If the issue was really zip codes, high schools, and so on, this shouldn't be the case.

The other thing this professor should've known is that high school is biased! GPAs are biased too!

The bias in GPAs has actually been exploited: elite high schools inflate grades and don't report class ranks, so students appear better than they are. Image
But you know what isn't a biased tool for admissions? Just one thing: test scores.

Want to learn more? Here are some sources:

x.com/cremieuxrecuei…

cremieux.xyz/p/what-happens…

cremieux.xyz/p/bias-in-admi…

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More from @cremieuxrecueil

Dec 18
College students make or are forced to make suboptimal choices about the times their classes take place🧵

For students who register for 8AM classes, about a third wake up after class starts, and almost 40% wake up too late to get to class on time. Image
People's internal rhythms aren't things they just choose, they're somewhat out of their control because they're synced up with day-night cycles.

Consider this, showing the amount of time 8AM class-takers sleep on school days vs weekends (gray), measured through logins at school. Image
If you compare those 8AM class-takers to 9AM students, you see that the ones who registered for 9AM classes sleep longer, but both sleep similar lengths on weekends. Image
Read 16 tweets
Dec 16
If you're curious about the recent rise in autism diagnoses, go read this.

It details how much of the rise in diagnoses is down to diagnostic drift and increasing screening. Image
You can see the impact of this on correlations between autism and other things in the published literature:
A common retort is 'But [this] study used the same definition over time and found an increase'. That comment is usually just wrong.

What people see in those studies is almost always a combination of 1. screening more, and 2. screening less stringently even if they don't want to.
Read 5 tweets
Dec 13
The potential gains to port automation are so enormous that Trump is making a huge mistake if he goes along with wishes of the mobsters in the ILA.

The gains on the table are so large that increasing an average port's capacity by just one ship increases total trade by 0.67%. Image
Bulk freight carriers also hate waiting around. They want to take goods and get them delivered where they need to go.

But America's ports are so inefficient that bulk carriers opt to go to the wrong ports to save time.

America's roads get sacrificed to its lack of automation. Image
Want to get automated ports and make America richer?

Destroy the ILA. Don't negotiate with them, just end them outright. The easiest way to do that is to make them happy with being destroyed.

Just Pay Them Off. Image
Read 4 tweets
Dec 12
The COVID era Paycheck Protection Program was defrauded at an incredible scale. People received PPP loans for total nonsense at stunning rates.

Thread of funny claims.

Dodge Hellcat LLCImage
Reparations for Indigenous People LLCImage
Just Traffic TicketsImage
Read 26 tweets
Dec 11
"A reminder: the US has the #1 most expensive healthcare system in the world, yet we rank roughly #42 in life expectancy."

Brian Thompson's murderer wrote that in his manifesto.

Both claims are bad. The first one, because America spends the most on healthcare because it's rich: Image
I don't mean Americans pay higher prices for the same amounts of care, but that Americans consume much higher volumes of care. They do more check-ups, get more screenings, take more tests, dose more drugs, get more surgeries... and so on!

You can predict spending from volumes:Image
Regarding the second claim, Americans have shorter lifespans because they're fat, violent, and reckless, not because of things that the health system can control.

And, if anything, when it comes to the things the health system actually controls, they generally do better!Image
Read 4 tweets
Dec 11
In Medicare Advantage, the government encourages insurers to pursue perverse incentives.

These see them overdiagnosing patients for conditions they often do not have, and which they don't recommend treating, because it means they can send the government a bigger bill. Image
Insurers have tried to claim that they don't do this, and that this data misrepresents the care they provide to patients they diagnose with particular conditions (like HIV!) because COVID disrupted care, but they're lying and it's too obvious. Image
Medicare Advantage does beat FFS on performance and on cost, and it certainly leads to more care for tons of people, but like every major program in medicine, it gets defrauded to the tune of tens of billions of dollars.

Oh well!
Read 4 tweets

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