Interpreting scatterplots
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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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Interpreting scatterplots

Interpreting Scatterplots

Presentation 2.3


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.


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.