PSY1022 Chapter Notes - Chapter Prescribed: Quasi, Random Assignment, Measuring Instrument
26 May 2018
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PSY1022 – Readings – Week 8 – Quasi experimental and experimental research
- comparing two or more groups of score - experimental and non experimental
methods
- the relationship between variables is examined by using one of the variables to
define the groups and then measuring the second variable to obtain scores for
each group
- the experimental method
• goal is to demonstrate a cause and effect relationship between two
variables
• changing the value of one variable causes changes to occur in the second
variable
• manipulation
•
o the researcher manipulates one variable by changing its value
from one level to another
o a second variable is observed/measured to determine whether the
manipulation causes changes to occur
• control
•
o the researcher must exercise control over the research situation to
ensure that other, extraneous variables do not influence the
relationship being examined
- variables that researches must consider
• participant variables
•
o these are characteristics such as age, gender and intelligence that
vary from one individual to another
o researchers must ensure that participant variables do not differ
from one group to another
• environmental variables
•
o these are characteristics of the environment such as lighting, time
of day and weather conditions
o a researcher must ensure that the individuals in treatment A are
tested in the same environment as the individuals in treatment B
- researchers typically use three basic techniques to control other variables
• random assignment
•
o each participant has an equal chance of being assigned to each of
the treatment conditions
o the goal is to distribute the participant characteristics evenly
between the two groups so no group is noticeably smarter etc than
the other
• matched participants
- experimental method
• one variable is manipulated while another variable is observed and
measured. to establish a cause and effect relationship between the two
variables, an experimenter attempts to control all other variables to
prevent them from influencing results
- independent variable
• the variable that is manipulated by the researcher. in behavioural
research the independent variable usually consists of the two or more
treatment conditions to which subjects are exposed. the independent
variable consists of the antecedent conditions that were manipulated
prior to observing the dependent variable
- dependent variable
• the one that is observed to asses the effect of the treatment
- control conditions in an experiment
• an experimental study evaluates the relationship between two variables
by manipulating one and measuring the other
• only one is measured
• different from a correlational study in which both variables are measured
and the data consist go two separate scores for each individual
• often an experiment will include a condition in which participants do not
receive any treatment
• no treatment condition is called the control condition
• the treatment condition is called the experimental condition
- individuals in a control condition no not receive the experimental treatment.
instead they either receive no treatment or they receive a neutral, placebo
treatment, the purpose of the control is to provide a baseline for comparison
with the experimental condition
- between subjects design
• an experiment in which different subjects are assigned to each group
• different people serve in the control and experimental groups
• some experiments may involve the use of two experimental groups with
no control group
- post-test only control group design
• an experimental design in which the dependent variable is measured
after the manipulation of the independent variable
- pretest/posttest control group design
• an experimental design in which the dependent variable is measured both
before and after manipulation of the independent variable
• added advantage of ensuring that the subjects are equivalent at the
beginning of the study
• this precaution is usually not considered necessary if participants are
randomly assigned and if a large sample is used
• disadvantages - possibility of increasing demand characteristics and
experimenter effects
• subjects might guess before the post-test what is being measured in the
study
• more opportunity for experimenter influence with multiple testings
- solomon four-group design
• a design with four groups that is a combination of the post-test only
control group design and the pretest/post-test control group design
• disadvantages of each individual design are minimised and the
advantages are maximised
- control and confounds
• confound
•
o an uncontrolled extraneous variable or flaw in an experiment
o if a study is confounded it is impossible to say whether changes in
the dependent variable were caused by the independent variable
- internal validity
• the extent to which the results of an experiment can be attributed to the
manipulation of the independent variable rather than to some
confounding variable
• needs to be maximised
• threats to internal validity
•
o nonequivalent control group
o
▪ using random sampling and random assignment typically
counters this
o history effect
o
▪ a threat to internal validity in which an outside event that is
not part of the manipulation of the experiment could be
responsible for the results
▪ changes in the dependent variable may be due to historical
events that occur outside of the study
o maturation effect
o
▪ a threat to internal validity in which naturally occurring
changes within the subjects could be responsible for the
observed results
▪ occurs in research in which subjects are studied over a
period of time
▪ subjects mature physically, socially and cognitively
▪ using a control group with equivalent subjects will indicate
whether changes in the dependent variable are due to
maturation
o testing effect
o
▪ a threat to internal validity in which repeated testing leads
to better or worse scores
▪ repeated testing may lead to better or worse performance
▪ practice effect
▪
▪ repeating the same test - increasing familiarity
▪ fatigue or negative practicing effect
▪
▪ repeated testing fatigues the subjects and their
performance declines as a result
▪ control group of equivalent subjects will help control for
testing confounds
o regression in the mean
o
