PLACEBO AND EXPERIMENTER
EFFECTS
Don’t trust your judgement – trust empirical research
Example of Research
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I want to test the effect of Melatonin on Insomnia
VARIABLES
 The Independent Variable is Melatonin (this is what I want to
see the effect of, what I am going to vary or manipulate, I
am going to get people to take melatonin or not take
melatonin)
 The Dependent Variable is Insomnia (this is what I am going
to measure to see any effect the IV has had, I am going to
measure the quality of participants sleep(who have
previously had trouble sleeping), I am going to measure how
many hours of uninterrupted sleep they have after taking or
not taking melatonin)
Experimental and Control Groups
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Group One is the Experimental group – they were
exposed to the IV (taking melatonin)
Group Two is called the Control group – they were not
exposed to the IV (not taking melatonin)
Now I can compare the difference in quality of sleep of
people who took melatonin Vs those who did not.
Extraneous variables….
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Lets assume these have been controlled
Our sample is stratified and large
We have used a matched participants design
We have operationalised our hypothesis
The Placebo Effect
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The placebo effect occurs when a participant's behavioural response
is influenced by their expectation of how they should behave
The expectations of the participants rather than (or as well as) the
independent variable, may be affecting the dependent variable
and therefore the results
For example, the participants in the above study think that they have
had a better nights sleep after taking melatonin. However their
experience of having a better nights sleep may have been caused by
their expectation that the melatonin would make them sleep better
In this case feeling like they slept better is a result of the belief that
they will sleep better not a result of the melatonin (IV) itself.
Controlling the Placebo Effect
Single Blind Procedure
 A single blind procedure involves allocating participants to groups so that
they are "blind" to the knowledge of which group they are in: experiment
or control
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For example in the study mentioned above I could give both groups
(experimental and control) a pill to take. One type of pill will contain
melatonin, the other will be a sugar pill with no active ingredients.
Now participants don’t know which pill they have taken (melatonin or sugar)
thus they do not know what group they are in (experiment or control) and
their behaviour is therefore not likely to be influenced by their expectations!
The Experimenter Effect
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The experimenter effect refers to the actions of the
experimenter, rather than (or as well as) the
independent variable, affecting the dependent
variable, and therefore the results of the experiment
The experimenter effect can take two forms
The Experimenter Effect
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Self-fulfilling prophecy - participants behave in line with the way
they believe the experimenter wants them to behave because the
experimenter unintentionally suggests to the participants how to
respond
For example in the study mentioned above the fact that the
experimenter knows that it was jimmy who took the melatonin may
cause the experimenter to unintentionally treat jimmy differently
thereby effecting Jimmy’s responses
The Experimenter Effect
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Experimenter Bias - unintentional actions that may
occur in the collection and treatment of
experimental data. E.g. Wrongly interpreting a
participant's response
So the experimenter’s expectation that the IV
(taking melatonin) will be effective has influenced
their judgement of the participants responses
Controlling the Experimenter Effect
Double-blind procedure
 The allocation of participants to groups so that both the
experimenter and the participants are "blind" to the
conditions to which the participants have been
allocated
 For example in the study mentioned above I could get
research assistants to hand out the pills so my
experimenters don’t know who is in which group
OTHER RESEARCH DESIGN
PROBLEMS
Now what else can go wrong?
Demand Characteristics
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How the research is conducted could alter how participants
behave
Social research is particularly vulnerable to this effect
People don’t respond ‘naturally’ rather they respond in line
with what they perceive as the required outcome of the
experiment
Participants do their best to be ‘good participants’
They do things they think the experiment requires, not what
they would usually do!
Demand characteristics can also be related to the way the
experimenter communicates with participants (subtle hints) or
how the research is set up.
Are people really responding spontaneously or are the
altering their behaviour to do what they think is ‘right’?
Artificiality
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Simply being in a lab can change the way people
respond
Trying to study ‘real life’ while in the lab is a
problem
Can limit the ability to generalise from the lab
results
Non standardised procedures
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Not all participants are treated the same or receive
identical instructions or conditions
A source of extraneous variables
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Researchers always try to avoid this at all costs!
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CASE STUDIES IN PLACEBO
AND EXPERIMENTER EFFECT
Homeopathy and the Power Balance bracelet
Homeopathy and Power Balance
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Both make claims about health and wellbeing
Both conduct ‘research’ to back their claims
Both show no understanding of the empirical/scientific
method
Clients of each experience the placebo effect
Researchers in each area suffer the experimenter effect
These people mean well, they are not lying, they simply
do not understand how to do good research