P.Mean: A fishy story about randomization (created 2012-05-12).

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I was told this story but have no way of verifying its accuracy. It is one of those stories that if it is not true, it should be. It illustrates why randomization is important. A long, long, time ago, a research group wanted to examine a pollutant to find concentration levels that would kill fish. This research required that 100 fish be separated into five tanks, each of which would get a different level of the pollutant.

Fish in a large tank about to be distributed to five smaller tanks

The researchers caught the first 20 fish and put them in the first tank,

Fish distributed to the first small tank

then put the next 20 fish in a second tank,

Fish next distributed to the second small tank

the next 20 in the third tank,

Then fish were distributed to the third small tank

the next 20 in the fourth tank,

Then fish were distributed to the fourth tank

and the last twenty in the fifth tank.

Finally, the remaining fish were distributed to the fifth tank

When they were done, they found that mortality was related not to the chemical concentration but to the order in which the tanks were filled. Most of the fishwere floating lifeless at the top of the first tank, but the fifth tank had almost all happy little swimmers.

Mortality was highest in the first tank, and lowest in the fith tank

What happened was that the slow-moving, easy-to-catch fish (the weakest and most sickly) were all allocated to the first tank. The fast-moving, hard-to-catch fish (the strongest and healthiest) ended up in the last tank.

Creative Commons License This page was written by Steve Simon and is licensed under the Creative Commons Attribution 3.0 United States License. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Randomization In Research.