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The following examples illustrate the use of controls in a variety of experimental situations;

• Presence or absence of a factor.
   

  Hypothesis:	Presence of fertiliser X promotes growth of plant species Y.
  Control group:	the absence of fertiliser X
  Experimental group:	has fertiliser X

• Range of factor being investigated.
   

  Hypothesis:	An increasing dose of dietary supplement would increase the milk production in cows.
  Control group:	0 amount of dietary supplement (no dose)
  Experimental group:	varying levels of dietary supplement

   

• Range of factor being investigated, where the factor cannot be excluded.
   

  Hypothesis:	A plant species C grows best in acid soils.
  Control group:	As it is not possible to have no pH, select one experimental group as the control and compare all others with that (In this case, pH = 7)
  Experimental group:	each group of plants would be grown at different pH’s

  
  

• Experiments involving humans and placebos.
   

  Hypothesis:	New drug D decreases blood cholesterol levels.
  Control group:	Patients given placebo
  Experimental group:	Patients being administered with drug D.

  To ensure that humans are not just psychologically responding to the drug or chemical some humans in the trial are unknowingly given a
  
  
  In addition the experiment could be -
  
  
  

• Experiments involving mock procedures. 
   

  Hypothesis:	Docking the tails of newborn lambs decreases growth rate in the first month of life.
  Experimental group:	Lambs separated from mother and rubber rings placed at base of tail (which restricts blood flow causing the tail to die and fall off)
  Control group:	lambs separated from mother and painful stimulus applied to tail (short term).

  This attempts to ensure both groups experience the same level of stress, as stress may affect growth.

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