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How Groups can effect 'r'^{} Groups can cause a relationship to be stronger, weaker, or have little effect. Investigate the influence of groups. If there are groupings in your data that behave differently then consider fitting a different linear regression line for each grouping to test the influence of the group.

Effects of Groups Booklet pg12

How Unusual Values can effect 'r' The relative position of an Unusual Value han have a huge effect on the 'relationship' Investigate the affect of Unusual Value and the robustness of the model. They may increase the strength of any relationship Remember the CONTEXT!!!! A good approach to modeling bivarate data is to complete the analysis including unusual values, and also analyse the data without unusual values. A comparison of the two models can be done, and a discussion of the significance, nature and appropriateness of the unusual values can be made. 
Effects of Unusual Values The Effect of Unusual Values http://illuminations.nctm.org Guessing http://istics.net Class Exemplar: Class Exemplar: Booklet pg9, 10, 11 
The effect of GroupsOverlapping Groups 

Groups causing a Stronger 'Relationship'Groups causing a Weaker 'Relationship'
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The effect of Unusual Values


Activity: 