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Contents Scatter graphs
· Scatter graphs, types of correlation and
lines of best fit
· Product­moment correlation coefficients
· The effects of coding on correlation
· Regression
* of 58 © Boardworks Ltd 2005…read more

Slide 3

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There are many situations where people wish to find out
whether two (or more) variables are related to each other.
Here are some examples:
· Is systolic blood pressure related to age?
· Is the life expectancy of people in a country related
to how wealthy the country is?
· Are A-level results related to the number of hours
students spend undertaking part-time work?
· Is an athlete's leg length related to the time in which
they can run 100m?
Correlation is a measure of relationship ­ the
stronger the correlation, the more closely related
the variables are likely to be.…read more

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Scatter graphs
Scatter graphs are a useful visual way of judging whether a
relationship appears to exist between two variables.
Example: The City Latitude Mean Jan. temp. (°C)
table shows the Belgrade 45 1
latitude and Bangkok 14 32
mean January Cairo 30 14
temperature (°C) Dublin 50 3
for a sample of Havana 23 22
10 cities in the Kuala Lumpur 3 27
northern Madrid 40 5
hemisphere. New York 41 0
Reykjavik 30 ­1
Tokyo 36 5…read more

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Scatter graphs
The data in the table can be presented in a scatter graph:
This shows that mean January temperature tends to
decrease as the latitude of the city increases. We say that
the variables are negatively correlated.…read more

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Scatter graphs
In this example, a city's temperature is likely to be dependent
upon its latitude ­ not the other way around. Temperature
cannot affect a city's latitude.
The latitude is called the independent (or explanatory)
variable. The temperature is called the dependent
(or response) variable.
When plotting scatter
graphs, the convention is
to always plot the
independent variable on
the horizontal axis and the
dependent variable on the
vertical axis.…read more

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A really good presentation of the formulae ( with worked examples) used in correlation and regression, includes scaling and reference to outliers.

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