Part I: A hypothetical cohort study

You are a dentist interested in studying the time between getting a normal filling in a molar (a relatively minor procedure) and needing root canal therapy for that molar (a more advanced procedure).You enroll a cohort of 100 consecutive individuals from your dental practice who had a filling placed in one of their molars.At every 6-month check-up they are counted as still enrolled in the study. You count the number of enrolled patients who had to seek root canal therapy in each 6-month interval after the initial filling - either at your office or with an endodontist (a dentist who specializes in the procedure) with a referral from your office.Over 18 months of study, 43 patients obtained a root canal and 26 patients either left your practice or were lost to follow-up.You begin to construct a life table with your findings:I

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