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Observational and Experimental Units

They are different.
The observational unit may be a sample from the experimental unit.

If animals in a certain experiment are divided into pens, where:

  • The animals in different pens receive different treatments
  • The animals in the same pen receive the same treatment
  • The treatments in different pens are administered independently

then the pen is the experimental unit. The animals in the pen are the observational units.

Note that when tabulating the results, each animal results in an observation, which is a table row.