PSY294 Statistics for Psychology

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PSY294 Statistics for Psychology
News headline
A report by the Chief Inspector of Schools
has found that there is no link between class
size and educational attainment
Actual Report
Among children aged 5-7 there is some
evidence that class size has an effect,
however, for children aged 8 and over, there is
no statistically significant evidence of a
correlation between class size and educational
attainment
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PSY294 Statistics for Psychology
Correlation
Correlation measures the strength of a
relationship between two measurements on a scale
from -1 to 1. The values 1 and -1 relate to a
perfect linear relationship between the
measurements The value 0 relates to no
association between the measurements. Correlation
is only sensible when a scatterplot indicates a
linear trend.
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PSY294 Statistics for Psychology
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PSY294 Statistics for Psychology
Correlation
Calculations (X is IQ and Y is Grade Point
Average)
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PSY294 Statistics for Psychology
Correlation
Leading to


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PSY294 Statistics for Psychology
Correlation
A test of the significance of r is a test of the
null hypothesis of no association between the
variables X and Y
Refer to Table 6.2 in Neave with n12.
We have p lt 0.002 - very highly significant
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PSY294 Statistics for Psychology
Correlation
Minitab (data is available on network drive
Q) MTBgt RETRIEVE Q\psy\gpdata.mtw or use
File menu command Open Worksheet
Put X and Y values in two columns, say C1 and C2
MTBgt CORRELATION C1 C2
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PSY294 Statistics for Psychology
Storks and birthrate in Denmark
If you examine the records of the city of
Copenhagen for the ten or twelve years
following World War II, you will find a
strong positive correlation between (i) the
annual number of storks nesting in the city, and
(ii) the annual number of human babies born in
the city.
WHY?
Association does not imply causality!
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PSY294 Statistics for Psychology
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PSY294 Statistics for Psychology
Spearmans rank correlation
Refer to Table 6.4 in Neave with n8.
Result is significant at 10 (p lt 0.1), retain
null but consider further investigation.
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PSY294 Statistics for Psychology
Kendalls rank correlation
21 concordant 7 disconcordant

• 14/28 0.5
• Use Table 6.5
• in Neave