Correlation Confusion! Watch

Moonspace
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In the S3 Edexcel Book there are two types of correlation , Spearmans and PMCC. For spearmans its supposed to be no tied ranks but in Example 2 in the book it says: During a cattle show two judges ranked ten cattle for quality according to the following table and then it goes on to solve the question by spearmans. How is this possible considering there are tied ranks.
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DFranklin
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You can use Spearman with tied ranks; the most common approach is to replace the tied ranks by the "average" rank they span.

i.e. if you 1, 2, 3, 3, 3 (tied), 6, 7, 8 so that there was a 3 way tie for 3rd place, then the 3rd place ranks are actually spanning positions 3, 4 and 5, so you'd replace 3 by the average of 3, 4 and 5.

The specification actually says; "Numerical questions involving ties will not be set. Some understanding of how to deal with ties will be expected."

I confess I don't really know what "some understanding..." means here...
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Moonspace
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(Original post by DFranklin)
You can use Spearman with tied ranks; the most common approach is to replace the tied ranks by the "average" rank they span.

i.e. if you 1, 2, 3, 3, 3 (tied), 6, 7, 8 so that there was a 3 way tie for 3rd place, then the 3rd place ranks are actually spanning positions 3, 4 and 5, so you'd replace 3 by the average of 3, 4 and 5.

The specification actually says; "Numerical questions involving ties will not be set. Some understanding of how to deal with ties will be expected."

I confess I don't really know what "some understanding..." means here...
I get how to work out ranks for the spearmans but in a question how would I know whether to use PMCC or spearmans. In the edexcel s3 book, in the first example in this chapter, the question says that two tea tasters were asked to rank blends of tea in oder of preference and then goes on to show the table. This question is solved with PMCC but the question in example 2, (written in initial post) which is similar to question 1 is done with spearmans and not PMCC
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DFranklin
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(Original post by Moonspace)
I get how to work out ranks for the spearmans but in a question how would I know whether to use PMCC or spearmans. In the edexcel s3 book, in the first example in this chapter, the question says that two tea tasters were asked to rank blends of tea in oder of preference and then goes on to show the table. This question is solved with PMCC but the question in example 2, (written in initial post) which is similar to question 1 is done with spearmans and not PMCC
Key is kind of in the name: if you're dealing with rankings you'd normally use a rank correlation coefficient such as Spearmans, if you're dealing with numbers (i.e. things that come from actual measurements etc) you'd normally use PMCC.

Note that you don't "have" to do it this way, so an example in a chapter introducing Spearman's RCC might well first use PMCC as a "compare and contrast"...
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Mr M
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(Original post by DFranklin)
I confess I don't really know what "some understanding..." means here...
I don't know either. I don't teach Edexcel or any Statistics modules.
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