From the table below, imagine that all the cases in a case-control study had their blood pressure measured by a single doctor at the local hospital but, for practical reasons, the controls had their blood pressure measured by their local doctor.

From the table below, imagine that all the cases in a case-control study had their blood pressure measured by a single doctor at the local hospital but, for practical reasons, the controls had their blood pressure measured by their local doctor. In this situation, it is likely there would be less random error in the blood pressure readings for cases that came from a single doctor than in those for controls that came from several different doctors.

	Raw			Misclassified
High BP		Cases	Controls		Cases	Controls
	Yes	300	250	Yes	300
	No	100	150	No	100

i. Calculate the OR of the raw table and re-calculate the OR if the measurement of exposure among cases was perfect but 20% of exposed controls were randomly misclassified as unexposed and vice versa.

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ii. Is this misclassification differential or non-differential and why?

iii. What effect has it had on the OR and why?

iv. What would the effect have been if we had misclassified cases instead of controls?

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