BIOL 1010 Lecture Notes - Internal Validity, Random Assignment, Statistical Significance

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30 Jan 2013
When the sign of r is negative, variables are said to be negatively related as scores on one
variable increase those for the other tend to decrease. Ex: # of hours spent watching t.v is
negatively correlated with grade point average
Scatter diagrams of positive and negative correlations
In perfect relationships all points fall on a straight line
The values tend to scatter increasingly and become dispersed as the correlation becomes lower
When the correlation reaches 0.00 knowledge of a persons score on one variable tells us
nothing about his or her score on the other
Statistical significance
The magnitude of a correlation coefficient tells us the strength of a relationship between two
Statistical significance refers to the likelihood that the results of an investigation are due to
A statistical significance correlation is one that is not likely to have occurred by chance
A correlation is considered statistically significant if the likelihood or probability that it is a
chance finding is 5 or less in 100. this level of significance is called the .05 level, commonly
written as p =0.5 (p stands for probability)
In general as the size of the correlation coefficient increases the result is more and more likely
to be statistically significant
Whether a correlation attains statistical significance depends on the # of observations made.
The greater the # of observations the smaller r (the correlation) needs to be to reach statistical
Applications to psychopathology
Whenever we compare ppl given one diagnosis with those given another or with ppl without a
psychological diagnosis the study is correlational
When the correlational method is used in research on psychopathology one of the variables is
typically diagnosis
To calculate a correlation between this variable and another one diagnosis is quantifies so that
having an anxiety disorder is designated by a score of 1 and not having one is score of 2
The diagnosis variable can then be correlated with another variable such as the amount of stress
that has been recently experiences
Variables such as having anxiety disorder or not are called classificatory variables
other ex are age, sex, social class and body build. These variables are naturally occurring
patterns and are not manipulated
most research on the causes of psychopathology is corrleational
problems of causality
correlatioanl method has a drawback: it does not allow determination of cause-effect
a sizeable correlation between 2 variable tells us only that they are related or tend to co vary
with each other but we do not really know which is cause and which is effect or if either variable
is actually the cause of the other
The directionability problem ---ex: a correlation has been found between the diagnosis of shizto
and social class: lower class ppl are more frequently diagnosed as having schitzo
It may be that the disorganized beh patterns of ppl with shitzo cause them to perform poorly on
their educational and occupational endeavours and thus to become impoverished.
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