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An extraneous variable is any factor which could influence the behaviour of
participants and influence a change in the dependent variable.
Types of extraneous variables
Participant (intelligence, age, gender)
Situational (temperature, instructions, time of day)
Demand characteristics occur when the participant guesses what the
investigation is about and changes his/her behaviour accordingly.
The likelihood of demand characteristics can be reduced by passively
deceiving the participant and using a single blind design (when you deceive
your participants, you don't tell them your hypothesis.
An investigator effect occurs when the researcher subconsciously
influences the behaviour of the participants. E.g. clever Hans
The likelihood can be reduced by using a double blind technique (not letting
the investigator conduct the study)
Confounding variables and internal validity
If extraneous variables do effect the dependent variable, then they are
called confounding variables. This means that the study will lack internal
validity as the results are not caused by the independent variable but by a
Example- Curtiss believed that Genie's developmental impairments were
caused by her early childhood privation. However her father thought she
was mentally retarded from birth due to her being brain damaged.