Download Definitions of Abnormality

Definitions of Abnormality
 Defining a person or behaviour as ‘abnormal’ implies
something undesirable and requiring change
 Therefore, we must be careful how we use the term
 Psychologists need methods for distinguishing
‘normal’ from ‘abnormal’
Definitions of Abnormality
 Our definition of abnormality must be objective:
 It must not depend on anyone’s opinion or point of view
 It should produce the same results whoever applies it
 It must not be under- or over-inclusive
 It must not label as ‘abnormal’ or ‘normal’ behaviours or traits
that aren’t
Definitions of Abnormality
 There are many ways of thinking about
normality/abnormality
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Statistical infrequency
Deviation from social norms
Deviation from ideal mental health
Failure to function adequately
Statistical Infrequency
 Under this definition, a person’s trait, thinking or
behaviour is classified as abnormal if it is rare or
statistically unusual.
 With this definition it is necessary to be clear about
how rare a trait or behaviour needs to be before we
class it as abnormal
Normal distribution
 We can work out whether a person’s
thinking/behaviour is statistically
abnormal by using standardized tests
which gather quantitative data
 Most psychological variables have
what is known as normal distribution
 This means that if you take a large
enough random sample, you will find
that when you plot the scores for most
psychological variables, you will end
up with a bell curve or normal
distribution curve
 Roughly 50% of people score above the
mean and 50% score below the mean
So, how do we work out who is abnormal?
 Having calculated the mean (a
measure of central tendency), we
next need to work out the standard
deviation, a measure of dispersion
which tells us on average how much
the scores in our sample differed
from the mean
 Most people, (approx 95%) score
between one and two standard
deviations above or below the mean
 This leaves just over 5% of people
who score unusually high or low
(more than or less than 2SDs from
the mean) and these people would
be classed as abnormal because
they are not typical of most people
frequency
Statistical Infrequency
A very small subset of
the population (<2.2%)
have an IQ below 70pts.
Such people are
statistically rare. We
regard them as having
abnormally low IQs
70
100
IQ Scores
130
Z scores: exactly how abnormal am I?
 Statisticians use Z scores to indicate how many standard
deviations an individual’s score lies above or below the mean
for any specific psychological variable
 for example ...
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a z score of +1 would suggest that the person had a score which was
above the mean but within one standard deviation
a z score of +2 would mean they were above the mean but within 2
standard deviations
a z score of +3 would mean they had an abnormally high score which
was in excess of 2 SDs above the mean
 Likewise...
 a z score of -1 would suggest that the person had a score that below the
mean but within one SD
 a z score of -2 would mean they scored upto 2SDs below the mean
 a z score of -3 would mean they had an abnormally low score which was
in excess of 2 SDs below the mean