Question 3: What does the correlation coefficient tell us about the agreement between capillary glucose and laboratory glucose?
The correlation coefficient tell us nothing about the agreement between capillary glucose and laboratory glucose. It tells us how closely they are related. A correlation r = 0.98 tells us that the two are closely related, but completely ignores the overall bias, an average difference = 0.36 mmol/L, which was statistically significant from 0, P = 0.007. Furthermore, this bias depends on the magnitude of the glucose. For small glucose, capillary glucose overestimates relative to laboratory glucose, for large glucose capillary glucose underestimates relative to laboratory glucose.
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