Table 1.

The classic contingency.

Exposure	Outcome (disease=D+) for example Tumor recurrence	No outcome (no-disease=D-) for example No tumor recurrence	Total
Intervention (exposed=E+) for example Dr. JJ’s new approach	Exposed with outcome =a (recurrent cases operated via Dr. JJ’s approach)	Exposed without outcome=b (non-recurrent cases operated via Dr JJ’s approach)	All exposed persons =a+b (all patients operated via Dr. JJ’s approach)
No intervention (non-exposed=E-) for example sub-frontal approach	Not exposed with outcome =c (recurrent cases operated via subfrontal approach)	Not exposed without outcome=d (non-recurrent cases operated via subfrontal approach)	All non-exposed persons =c+d (All patients operated via the subfrontal approach)
	All persons with outcome =a+c (all patients with tumor recurrence)	All persons without outcome =b+d (All patients without tumor recurrence)	All persons a+b+c+d

Risk or incidence of recurrence= a/a+b (for Dr. JJ’s approach) & c/c+d (for subfrontal approach), relative risk (RR)=[Risk in exposed]/ [Risk in the non-exposed]=[a/a+b]/ [c/c+d], odds of exposure in the cases=[a/c] and odds of exposure in the controls=[b/d], odds ratio (OR)=[odds of exposure in the cases]/[odds of exposure in the controls]=[a/c]/[b/d]