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In the terminology of that we have developed, John Snow conducted anobservational study, not a randomized experiment. But he called his study a“grand experiment” because, as he wrote, “No fewer than three hundred thousandpeople … were divided into two groups without their choice, and in most cases,without their knowledge …”

Studies such as Snow’s are sometimes called “natural experiments.” However, truerandomization does not simply mean that the treatment and control groups areselected “without their choice.”

The method of randomization can be as simple as tossing a coin. It may also bequite a bit more complex. But every method of randomization consists of asequence of carefully defined steps that allow chances to be specifiedmathematically. This has two important consequences.

  • It allows us to account–mathematically–for the possibility that randomizationproduces treatment and control groups that are quite different from eachother.

  • It allows us to make precise mathematical statements about differencesbetween the treatment and control groups. This in turn helps us makejustifiable conclusions about whether the treatment has any effect.

In this course, you will learn how to conduct and analyze your own randomizedexperiments. That will involve more detail than has been presented in thissection. For now, just focus on the main idea: to try to establish causality,run a randomized controlled experiment if possible. If you are conducting anobservational study, you might be able to establish association but notcausation. Be extremely careful about confounding factors before makingconclusions about causality based on an observational study.

Terminology

  • observational study
  • treatment
  • outcome
  • association
  • causal association
  • causality
  • comparison
  • treatment group
  • control group
  • epidemiology
  • confounding
  • randomization
  • randomized controlled experiment
  • randomized controlled trial (RCT)
  • blind

  • placebo Fun facts

  • John Snow is sometimes called the father of epidemiology, but he was ananesthesiologist by profession. One of his patients was Queen Victoria, whowas an early recipient of anesthetics during childbirth.

  • Florence Nightingale, the originator of modern nursing practices and famousfor her work in the Crimean War, was a die-hard miasmatist. She had no timefor theories about contagion and germs, and was not one for mincing herwords. “There is no end to the absurdities connected with this doctrine,” shesaid. “Suffice it to say that in the ordinary sense of the word, there is noproof such as would be admitted in any scientific enquiry that there is anysuch thing as contagion.”

  • A later RCT established that the conditions on which PROGRESA insisted –children going to school, preventive health care – were not necessary toachieve increased enrollment. Just the financial boost of the welfarepayments was sufficient.

Good reads

The Strange Case of the Broad Street Pump: John Snow and the Mystery ofCholera by Sandra Hempel,published by our own University of California Press, reads like a whodunit. Itwas one of the main sources for this section's account of John Snow and hiswork. A word of warning: some of the contents of the book are stomach-churning.

Poor Economics, the best seller by Abhijit V.Banerjee and Esther Duflo of MIT, is an accessible and lively account of ways tofight global poverty. It includes numerous examples of RCTs, including thePROGRESA example in this section.

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