There are three types of extraneous variables; Participant, Situational & Investigator effects.
Participant variables
Personal characteristics of the participant
Demand characteristics
Participants are able to guess the aim of the study. It can influence participants behaviour.
Demand characteristics can lead to the Please-You, or Screw-You effect.
Situational and Experiementer variables can be solved through standardisation.
Standardisation
When researchers make an extraneous variable same for all participants in a study.
To deal with participant variables, a researcher can match participants on the basis of their characteristics. Another way is through random allocation.
Demand characteristics can be controlled through either single, or double-blind study.
Confounding Variables
Any variable, other than the IV, that could affect the DV. CV's change systematically with the IV
