Suppose that for every root canal case from the previous 18 months, 3 controls were selected and both cases (43) and controls (129) were asked the about various exposures.

No Root Root Conditional Conditional Cumulative Canals at Canal Censored Effective Probability Probability Interval During Number Probability of "Survival" Beginning of "Survival" During of Root of Interval Interval at Risk Interval Canal (No Root (No Root Canal) Canal) Months 100 10 12 1-6 Months 78 15 8 7-12 Months 55 18 6 13-18