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Chapter 9

PSYB01H3 Chapter Notes - Chapter 9: Nonprobability Sampling, Interpersonal Communication, Operational Definition

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Anna Nagy

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Chapter 9 – Conducting Experiments
Selecting Research Participants
The methods used to select participants has implications for generalizing the
research results
Recall most experiments involve sampling participants from the population of
interest, which is composed of all individuals of interest to the researcher
Samples may be drawn using probability sampling or non-probability sampling, but
when it is important to accurately describe the entire population you must use
probability sampling.
Much research, however, is interested in testing hypothesis about behaviour, so the
focus is the relationships between the variable being studied and testing predictions
made based on theories of behaviour. In such cases, participants may be found in
the easiest way possible – non-probability sampling. Nothing is wrong with this as
long as you recognize that this affects your ability to generalize your results to
larger populations, though it is still possible to
You also need to determine your sample size.
Increasing the sample size increases the likelihood that results will be statistically
significant because larger populations are better estimates of the true population
Manipulating the Independent Variable
To manipulate the independent variable you must construct an operational
definition of the variable – you must turn a conceptual variable into a set of
Also, the independent and dependent variables must be introduced within the
context of the total experimental setting which is known as “setting the stage
Setting the Stage
In setting the stage you must do two things:
1. provide the participants with the informed consent information needed for
your study
2. explain to participants why the experiment is being conducted
sometimes the rationale given is entirely truthful, but the researcher will rarely give
the hypothesis because if participants know exactly what you are studying they may
try to confirm the hypothesis.
Therefore, you may tell the participants you are studying memory when you are
actually studying a certain aspect of memory.
If deception is initially necessary, you have an obligation to address the deception
when you debrief the participants afterwards
There are no clear cut rules for setting the stage except the experimental setting
must seem plausible to the participants
Types of Manipulation

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Straightforward Manipulation
Researchers are usually able to manipulate a variable relatively simply by
presenting written, verbal, or visual material to the participants
Straightforward manipulation – manipulated variables with instructions and
stimulus presentations
Stimuli may be presented verbally, in written form, via video tape, or over the
Eg. A study on the impact of heath promotion brochures presented two types of
brochures to women – one with text only, and one with pictures
Most memory research relies on straightforward manipulations. Eg Langdon
displayed lists of words to participants and measured their recall. The word lists
differed in phonological similarity: some words sounded alike like “cat” and “hat”
and some did not. They found that lists with dissimilar words were recalled more
Good examples of straightforward manipulation on page 168
Most manipulations of independent variables are done straightforward – researchers
vary the difficulty of material to be learned, motivation, the way questions are
asked, etc, in a straightforward manner
Staged Manipulations
Less straightforward – sometimes it is necessary to stage events during the
experiment to manipulate the independent variable successfully.
Aka “staged or even manipulated”
Most frequently used for two reasons
1. to create some psychological state in the participants, such as frustration or
2. may be necessary to simulate some situation that occurs in real life, such as
studying cognitive performance under conditions of multiple task demand
staged manipulations frequently employ a confederate or an accomplice. Usually
the confederate appears to be another participant. They are useful in creating a
particular social situation.
Eg in a study on aggression, the confederate and the participant are both told to wait
in a room, where the confederate insults the participant in the “anger” condition but
does not insult the participant in the “no-anger” condition.
The classic experiment by Asch on conformity is an example of the use of
confederates. When participants were told to match a line to the line of the same
length, the multiple confederates unanimously gave the wrong answer. Because of
this, the participants conformed and gave the same, clearly wrong answer as the
Look at table 9.1 on page 170
Staged manipulations require a great deal of ingenuity and some acting ability on
the part of the confederate, for they are meant to involve the participants in an
ongoing social situation which they perceive to be real, not part of the experiment.
Researchers assume that the result will be natural behaviour on the part of the

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However, such experiments may be difficult to replicate because they involve a
great deal of interpersonal communication that is difficult to put into words
A complex manipulation is also difficult to interpret because if there were many
things happening during the experiment, what one thing was responsible for the
observed effect?
Strength of the Manipulation
A general principle to follow is to make the manipulation as strong as possible – a
strong manipulation maximizes the differences between groups and increases the
chance that the independent variable will have a statistically significant effect on
the dependant variable.
A strong manipulation is most important in the early stages of research when you
are trying to establish that a relationship does in fact exist at all. If the early
experiments reveal a relationship, subsequent studies can systematically manipulate
other levels of the independent variable to provide a more detailed picture of the
The principle of using the strongest possible strength of manipulation should be
tempered by at least two things:
1. the strongest possible manipulation may involve a situation that rarely, if
ever, occurs in the real world. Eg. An extremely strong crowding
manipulation – so strong that the participants can’t even move – may
significantly impact behaviour, but it is unknown if the results were similar
to those occurring in more common, less crowded situations in the real
2. ethics: a manipulation should be as strong as possible within the boundaries
of ethics. A strong manipulation causing fear or anxiety, for example, might
not be possible because of potential physical or psychological harm to
Cost of Manipulation
researchers who have limited monetary resources may not have the money to afford
expensive equipment, salaries for confederates, or payments to participants for
long-term studies.
Also, a manipulation that requires the experiment to be run on each individual takes
up more of the experimenter’s time than one that can be run on multiple
participants at a time. In this respect, a straightforward manipulation of verbal or
written material is less costly than a complex, staged experiment
Measuring the Dependent Variable
Types of measures
the dependent variable in most experiments is one of three types: self-report,
behavioural, or physiological
Self-Report Measures
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