Correlation, Causation and Thinking Things Through

Us teaching type folks love to say that correlation isn’t causation. As with most things in life, the trouble starts when you try to decipher what this means, exactly. Wikipedia has an entire article devoted to the phrase, and it has occupied space in some of the most brilliant minds that have ever been around.

Simply put, here’s a way to think about it: not everything that is correlated is necessarily going to imply causation.

For example, this one chart from this magnificent website (and please, do take a look at all the charts):

https://www.tylervigen.com/spurious-correlations

But if there is causation involved, there will definitely be correlation. In academic speak, if x and y are correlated, we cannot necessarily say that x causes y. But if x does indeed cause y, x and y will definitely be correlated.

OK, you might be saying right now. So what?

Well, how about using this to figure out what ingredients were being used to make nuclear bombs? Say the government would like to keep the recipe (and the ingredients) for the nuclear bomb a secret. But what if you decide to take a look at the stock market data? What if you try to see if there is an increase in the stock price of firms that make the ingredients likely to be used in a nuclear bomb?

If the stuff that your firm produces (call this x) is in high demand, your firm’s stock price will go up (call this y). If y has gone up, it (almost certainly) will be because of x going up. So if I can check if y has gone up, I can assume that x will be up, and hey, I can figure out the ingredients for a nuclear bomb.

Sounds outlandish? Try this on for size:

Realizing that positive developments in the testing and mass production of the two-stage thermonuclear (hydrogen) bomb would boost future cash flows and thus market capitalizations of the relevant companies, Alchian used stock prices of publicly traded industrial corporations to infer the secret fuel component in the device in a paper titled “The Stock Market Speaks.” Alchian (2000) relates the story in an interview:
We knew they were developing this H-bomb, but we wanted to know, what’s in it? What’s the fissile material? Well there’s thorium, thallium, beryllium, and something else, and we asked Herman Kahn and he said, ‘Can’t tell you’… I said, ‘I’ll find out’, so I went down to the RAND library and had them get for me the US Government’s Dept. of Commerce Yearbook which has items on every industry by product, so I went through and looked up thorium, who makes it, looked up beryllium, who makes it, looked them all up, took me about 10 minutes to do it, and got them. There were about five companies, five of these things, and then I called Dean Witter… they had the names of the companies also making these things, ‘Look up for me the price of these companies…’ and here were these four or five stocks going like this, and then about, I think it was September, this was now around October, one of them started to go like that, from $2 to around $10, the rest were going like this, so I thought ‘Well, that’s interesting’… I wrote it up and distributed it around the social science group the next day. I got a phone call from the head of RAND calling me in, nice guy, knew him well, he said ‘Armen, we’ve got to suppress this’… I said ‘Yes, sir’, and I took it and put it away, and that was the first event study. Anyway, it made my reputation among a lot of the engineers at RAND.

https://www.sciencedirect.com/science/article/abs/pii/S0929119914000546

I learnt about this while reading Navin Kabra’s Twitter round-up from yesterday. Navin also mentions the discovery of Neptune using the same underlying principle, and then asks this question:

Do you know other, more recent examples of people deducing important information by guessing from correlated data?

https://futureiq.substack.com/p/best-of-twitter-antifragility-via

… and I was reminded of this tweet:


Whether it is Neptune, the nuclear bomb or the under-reporting of Covid deaths, the lesson for you as a student of economics is this: when you marry the ability to connect the dots with the ability to understand and apply statistics, truly remarkable things can happen.

Of course, the reverse is equally true, and perhaps even more important. When you marry the ability to connect the dots with a misplaced ability to understand and apply statistics, truly horrific things can happen.

Tread carefully when it comes to statistics!

Links for 11th March, 2019

  1. “Well, I hope the ongoing changes in policy towards the Chinese government, most of which I think are justified as a direct response to Chinese government actions, do not also lead to a general prejudice against ordinary Chinese people or all things Chinese.So far, we haven’t seen that, at least not much.For example, Trump, who’s been utterly shameless in provoking racial and ethnic tensions when it comes to African-Americans, Latinos, Mexicans, Africans — maybe I’m missing something, but I haven’t seen the same sort of thing on China yet.Trump seems to put China mostly into the trade/jobs economic section of his brain, rather than the “chaos/social upheaval/white nationalism” section of his brain. (And that’s one reason why, so far, lots of Democrats and independents have supported his policies, along with the Trumpists.)”
    That second paragraph worries me a little bit, although I am unsure of my analysis. Economics and culture (very roughly, that’s how I think about the two concepts mentioned above) aren’t independent. The more I read about economics, the more I think each feeds upon the other, and that too, continuously. Such compartmentalization seems too simplistic. The rest of the interview is also worth reading – and as somebody who appreciates great questions, I loved the very last one.
  2. “In March 1951, a frustrated Kodak threatened to sue the U.S. government for the “considerable amount of damage to our products resulting from the Nevada tests or from any further atomic energy tests…” Finally the company and the government came to an agreement. The AEC would provide Webb, by now the head of Kodak’s physics division, with schedules and maps of future tests so that Kodak could take the necessary precautions to protect its product. In return, the people of Kodak were to keep everything they knew about the government’s Nevada nuclear testing a secret.”
    The world is stranger than you can know, and imagine. It is also scarily stupid in ways one simply couldn’t have contemplated. A sobering read about how Kodak discovered scary stuff about America’s nuclear bomb experiments – and was essentially asked to keep quiet about it.
  3. “Perhaps because most of us are descendants of immigrants thrust into an artificial construct of a nation, or maybe because we live in a country that is constantly renewing and rebuilding, one of the few tangible things that connects us to the past and our cultural identity is food.”
    Ten dishes you might want to try in Singapore, with a little bit of history thrown in. I am sad to report that I haven’t tasted all of them yet.
  4. “Many local African churches have reached out to Chinese workers, including incorporating Mandarin into services. A number of Chinese, in turn, have welcomed the sense of community and belonging that these Christian churches offer. And a small but growing number of ethnically Chinese missionaries from Taiwan and other countries are specifically targeting Chinese nationals in Africa, preaching to them with a freedom they’d never be allowed in the People’s Republic.”
    If the rest of the world is worried about Africa being unduly influenced by neocolonial China… China, it turns out, is worried about being influenced by evangelical Christianity from Africa.
  5. “If you missed reports of the shenanigans at Canada’s McMaster University last week, then the following article by academic Kevin Carrico is well worth a read. Universities are letting a minority of Chinese students behave in ways that are utterly unacceptable. One speculates that they do this because many universities depend heavily on Chinese students for fee income, because they and their academics fear the Chinese Communist Party, and because university administrations tend to be pretty weak-kneed.”
    I had linked a while back to events in Canada, at a university. Joe Studwell, author of the fantastic How Asia Works, links to an article that provides perspective on this issue.