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Interpreting Scatterplots. Presentation 2.3. Scatterplots. When looking at scatterplots, we will look for: Association (or direction) Form Strength Outliers. Direction. A trend that runs from the upper left to the lower right is said to have a negative association or direction.

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Presentation Transcript
scatterplots
Scatterplots
  • When looking at scatterplots, we will look for:
    • Association (or direction)
    • Form
    • Strength
    • Outliers
direction
Direction

A trend that runs from the upper left to the lower right is said to have a negative association or direction.

A trend running the from the lower left to upper right has a positive association or direction.

It is a lot like slope!

direction1
Direction
  • It is possible for a scatterplot to have no association or direction.
  • This occurs when the plot looks like a random splattering of dots.
slide5
Form
  • Form:
    • If there is a straight line (linear) relationship, it will appear as a cloud or swarm of points stretched out in a generally consistent, straight form.
    • If the relationship isn’t straight, but curves, while still increasing or decreasing steadily, we can often find ways to make it more nearly straight.
strength
Strength
  • Strength:
    • At one extreme, the points appear to follow a single stream (whether straight, curved, or bending all over the place).
    • At the other extreme, the points appear as a vague cloud with no discernable trend or pattern.
  • Note: we will quantify the strength soon (that’s the r).
outliers
Outliers
  • Outliers:
    • As before, points that do not follow the pattern.
    • Outliers may be in either the x-direction, y-direction, or both directions.
    • There’s no better example than the 2000 presidential election.
outliers1
Outliers
  • 2000 Presidential Election
    • Remember Florida
    • The scatterplot on the next page details the number of votes for Bush and the number of votes for Buchanon.
    • As you would expect, the more people voting for Bush should mean more people voting for Buchanon (since there are simply more people voting).
    • In Palm Beach County there was much confusion about the ballot.
      • The Democratic Party alleged that the ballot was poorly designed thus skewing the vote.
      • The scatterplot shows the evidence.
2000 presidential election1
2000 Presidential Election
  • Apparently in Palm Beach County only, a disproportionate number of people voted for Buchanon.
    • This point is a great example of an outlier.
      • The outlier is in the y-direction (vertical).
  • Why did this happen?
  • The next slide shows the ballot voters used.
scatterplots1
Scatterplots
  • Remember, when looking at scatterplots, look for:
    • Association (or direction)
    • Form
    • Strength
    • Outliers
scatterplots2
Scatterplots
  • This concludes this presentation.
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