” … we may then define intellect in general as the power of good response from the point of view of truth or fact.” – Thorndike, 1921
Edward Tufte impresses me. His books on visual data show him as possessing in copious amounts that very rare commodity – truly original thoughts . So, when he tweeted the other day that this paper by Hill was “probably the best paper ever about making causal inference about human behavior”, of course I had to read it.
This got me to wondering about how we know something is true and led me to another thing I have learned in (almost) 55 years.
#15: Just because you believe something passionately doesn’t make it true.
Sometimes it might. If you believe passionately that you can earn a Ph.D. , win a gold medal in the world judo championships, run a marathon or swim the English channel, perhaps you can make it true. However, no matter how much you like your cigarettes, no matter how strongly you believe that tobacco is good for you because it is “natural”, the data are not on your side.
When a correlation between two characteristics is observed, it is common for people who don’t want that relationship to exist to object,
“Correlation doesn’t prove causation”
That is completely true. That is also not the same thing as correlation being unrelated to causation. Correlation can provide SUPPORT for a hypothesis of causation, although it is true that it cannot provide proof. In other words, we have more confidence to believe that some things are good, more than others, from the point of view of truth or fact. Statisticians even quantify that degree of confidence in something called a confidence interval.
In his paper, Hill discusses the strength of the association found. If the death rate of the population of people in an area with a very high rate of air pollution is 14 times higher than in another area with a low rate of air pollution, then we have more confidence of a possible causal relationship than if it is 1.14 times higher.
He also discusses replication and consistency. If you can find one or two studies on a topic that support your belief, that doesn’t make it true. There is a lot more in Hill’s article. It’s both short and brilliant. You should read it.
My dissertation advisor, the late Dr. Richard Eyman, gave me a lot of profound advice. One piece of it was
When the results don’t come out the way you expect, check everything over again. Make sure your measures are reliable and valid. Check for outliers and re-run your analyses without them. Go over everything again and look for threats to the validity of your design – the treatment was administered as you expected, the tests were administered according to the standard procedures. Run your study again and see if you get the same results. And when your results do come out the way you expect – DO THE EXACT SAME THING!
Just because you believe it, doesn’t make it true.
For #14 of the things I have learned in (almost) 55 years, click here. I’m trying to get to 55 by the time I turn 55 in August. I believe I can do it!