control of variables

• extraneous variables


  any variable that effects the DV and does not vary systematically with the IV e.g. the weather
• cofounding variables 


  do change systematically with the IV and becomes difficult for the researcher to be sure of the origin of the impact of DV
• demand characteristics 


  any cue the researcher or research situation may give which makes the participant feel like they can guess the aim of the investigation
• investigator effects 


  refers to any unwanted influence from the researchers behaviour, either conscious or unconscious on the DV measured
• randomisation


  use of chance to reduce the effects of bias from investigator effects
• standardisation 


  using the exact same formalised procedures and instructions for every single participant involved in the research process