Confounding Variables

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Confounding Variables
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Definition

• Confounding variables are two variables
  (explanatory or lurking variables) that are
  confounded when their effects on a response
  variable cannot be distinguished from each other.

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Example

• A soccer coach wanted to improve the team's
  playing ability, so he had them run two miles a
  day. At the same time the players decided to take
  vitamins. In two weeks the team was playing
  noticeably better, but the coach and players did
  not know whether it was from the running or the
  vitamins.

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Case Study

• If, for example, subjects in one group are
  simultaneously tested in a room with the heat set
  at 70 degrees whereas subjects in another group
  are simultaneously tested in a nearby identically
  appointed room with the heat set at 60 degrees,
  the obtained differences in performance could be
  attributed to any of three factors.

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• It could be due to the random assignment of
  subjects (i.e. to chance). It could be due to the
  different temperatures in the two rooms. It
  could, however, be due to some confounding factor
  such as differences in ambient illumination that
  result from unnoticed differences in the
  orientation of each room with respect to the sun.

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• In any experiment an appropriate statistical
  test can help in the decision as to whether or
  not to attribute the results to chance, but only
  the most careful analysis of the actual
  conditions of the experiment can suggest whether
  or not the result might be due to a confounding
  factor.

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Lurking Variables
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Definition

• A lurking variable is a variable that has an
  important effect on the relationship among the
  variables in a study but is not included among
  the variables studied.

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Example

• There's this guy who's going to clean the windows
  of a mental asylum. A patient follows him shouts
  to him "I gotta secret, I gotta secret...", he
  ignores the patient. Again the patient follows
  him, but he ignores his cries. By the time he's
  nearly finished the building, he's really curious
  about what the patients secret is, so he decides
  to ask the patient.

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• The patient pulls a matchbox out of his pocket,
  opens it and puts it on a table. Out crawls this
  little spider. The patient says "spider go left",
  and the spider walks to it's left a bit. Then he
  says "spider go right", the spider walks to its
  right a little bit. He says "spider turn around,
  walk forward then go right", and sure enough the
  spider turns around, walks forward, and then goes
  right a bit. The window cleaner is amazed "Wow!
  He says, that's amazing!", "No, that's not my
  secret says the patient, watch".

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• He picks up the spider in his hand and pulls all
  its legs off then puts it back on the table.
  "Spider go right", the spider doesn't move,
  "spider go Left", the spider doesn't move,
  "Spider turn around" again the spider doesn't
  move. "There!" he says, "that's my secret, if you
  pull all a spiders legs off they go
  deaf....................