Scatter-graphs

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Scatter-graphs

• How to produce, describe and interpret a
  scatter-graph
• - Correlation
• - Line of Best Fit

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As one goes up...
The other goes up
Positive correlation
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As one goes up...
The other goes down
Negative correlation
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No correlation
Perfect positive correlation
Weak negative correlation
Weak positive correlation
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No correlation
No correlation
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• Line of Best Fit
• A line as close as possible to all points
• How?
• 1) The line should have an equal number of
  points either side of it.
• The line should go through
• the mean point
• (x-coordinate mean of all x values,
  y-coordinate mean of all y values)

Optional
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Line of Best Fit A line as close as possible to
all points Why? The line can be used to predict
values Interpolation Predicting values within
the range covered. Extrapolation Predicting
values outside the range covered.
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Line of Best Fit Post hoc, ergo propter
hoc After it, therefore because of it Rarely
true Just because two things are correlated
doesnt mean one causes the other.
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• Discuss
• If you spend a lot of time on the phone, you
  have a better chance of ending up in a
  high-paying job
• The larger your shoe-size, the greater your bank
  overdraft
• Learning maths makes you a better musician

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• Positive, Negative or None?
• Remember correlation doesnt mean one thing
  causes the other it just means there is a
  relationship between them.
• Time spent travelling ? Distance covered
• Level of education ? Level of income
• Amount of clothing worn ? Temperature outside
• Number of pages in a book ? Copies sold
• Cost of car insurance ? Age of driver
• Number of socks owned ? Number of cars owned

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