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What is A cardiologist found a highly significant correlation coefficient (r = 0.90, p = 0.01) between the systolic blood pressure values and serum cholestrol values of the patients attending his clinic. Which of the following statements is a wrong interpretation of the correlation co-efficiency observed? and explain in detail?
Correlation reflects the strength & direction of relationship between two quantitative measurements. It can be used to predict the trend of an association, (e.g. patients with high serum cholesterol will have high BP) but it is not necessary for independent magnitudes of both the measurement to be close to each other. In fact, the individual measurements may be widely different, yet, with a strong positive correlation, an increase in one measurement will cause an increase in the other. + Sign means that there is a positive correlation between blood pressure values & serum cholesterol values. 0.9 means association is strong (since 0.9>0.5) R = 0.9 hence r2 = 0.9 x 0.9 = 0.81 or 81% of the variance in blood pressure in this study accounted for by variance in serum cholesterol & vice versa. Ref: High Yield Biostatistics By Anthony Glazer, Page 47; Fundamentals of Biostatistics By Bernard Rosner, 2010, Page 452