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-1    = Perfect Negative Correlation1    = Perfect Positive Correlation

-0.8 = Good Negative Correlation0.8 = Good Positive Correlation

-0.5 = Some Negative Correlation0.5 = Some Positive Correlation

0 = No Correlation

A line of best fit should only be drawn on a scatter graph if the correlation coefficient is >0.6 or <-0.6

The reason for displaying the equation of line of best fit is that it can be used to make predictions.  E.g. If the equation for Year 8 boys is y = 50x-40 this means for a boy in Year 8 his weight can be predicted if you know his height from calculating

weight = 50xheight – 40.

r2 (called R2 in Excel) is the square of the correlation coefficient and allows you to look at the likelihood of obtaining correct predictions from a line of best fit.  R2 is the likelihood that an increase in x will produce an increase in y. (i.e. that an increase in height will mean an increase in weight).

E.g. If the Correlation coefficient, r = 0.8

then r2 = 0.64

= 64%

That is a 64% chance that from any point on the line increasing the height will result in an increase in weight.

To Draw a Scatter Graph in Excel:

1.

Highlight the two columns of data

2.

Click on Chart Wizard (Bar chart icon on tool bar)

3.

Choose XY(Scatter)

4.

Enter chart title and label axes(remember units!)

5.

In Legend untick box labelled Show Legend

6.

Choose whether to save as separate chart or on sheet

To Improve Presentation:

Right click on x-axis and select format axis, choose scale and change minimum value.  Can repeat for y-axis if necessary.

To Put on a Line of Best Fit (only if strong enough correlation):

Right click on a point in the scatter graph, select add trendline.  In options tick boxes to display equation and R-squared on graph.

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