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Exam II Marks. Chapter 20.1 Correlation Correlation is used when we wish to know whether two randomly distributed variables are associated with each.

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Presentation on theme: "Exam II Marks. Chapter 20.1 Correlation Correlation is used when we wish to know whether two randomly distributed variables are associated with each."— Presentation transcript:

1 Exam II Marks

2 Chapter 20.1 Correlation

3 Correlation is used when we wish to know whether two randomly distributed variables are associated with each other Example – Total length Y1 of aphid stem mothers and mean thorax length Y2 of their parthenogenetic offspring.

4 No causal ordering

5 Contrast to regression

6

7 Formal model Regression randomly distributed response variable ~ fixed explanatory variable Correlation two random response variables No causal ordering, thus no explanatory variable

8 Compute t

9 State H A /H o pair Crunch the numbers

10 More number crunching > cor.test(dat$th.length,dat$tot.length) Pearson's product-moment correlation data: dat$th.length and dat$tot.length t = , df = 13, p-value = alternative hypothesis: true correlation is not equal to 0 95 percent confidence interval: sample estimates: cor

11 Conclusions r = 0.650, n = 15, p = Thorax length of offspring is positively related to stem mother total length. The relation of offspring thorax length to size of aphid stem mothers is monotonic but not necessarily linear.

12


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