control of variables

• extraneous variables 


  another variable (other than the IV) that may have an effect on the DV
• cofounding variables 


  another variable (other than the IV) that may affect the DV
• demand characteristics 


  participants changing their behaviour within the research
• investigator effects


  any effect of the investigators behaviour on the research
• randomisation 


  use of chance in order to control the effect bias when designing materials and deciding the order of conditions
• standardisation 


  using exactly the same formalised procedures and instructions for all participants