Marie_Senatus_Homework4

Exercise 4 I, Marie Senatus________affirm that I have completed this assignment in accordance with the Code of Academic Integrity and understand that all work done outside of the Live Sessions is to be done independently, without collaboration with other students, tutors or referring to past student exercises from this course. In addition, I understand that this exercise is not to be copied, shared, or uploaded to any online sites, with or without answers, in whole or in part without instructor permission. This exercise is graded out of a total 50 points and is worth 10% pf your final grade. It is due 3 days after LS8. Please answers questions clearly and show your work to facilitate any partial credit if you have the wrong numbers! Check with your section instructor as to the best format for submission. Read each question carefully. After working on it as a group complete your work independently but you can still reach out to your instructor for clarification and help on any concepts. 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 At every 6-month check-up they are counted as still enrolled in the study or if as withdrawn if they are no longer coming in to this practice. 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: IN PREP Interval No Root Canals at Beginning of Interval Root Canal During Interval Withdrew During Interval Effective Number at Risk Conditional Probability of Root Canal Conditional Probability of “Survival” (No Root Canal) Cumulative Probability of “Survival” (No Root Canal) Months 1-6 100 16 12 100 – (1/2)* 12 = 94 16/94=0.17 1- 0.17=0.83 0.83 Months 7-12 100-16- 12=72 15 8 72 – (1/2)* 8 = 68 15/68=0.22 1- 0.22=0.78 0.78*0.83=0.647 Months 13-18 72-15- 8=49 12 5 49 – (1/2)* 5 = 46.5 12/46.5=0.2 6 1-0.26= 0.74 0.74*0.647=0.48 0 PubH 6003_U: Principles and Practice of Epidemiology Exercise 4

a. Fill in the above table (9 pts) b. Based on the life table, what was the probability that a patient had received root canal therapy within one year of receiving a filling? (3 pts) IN PREP P(Root Canal within 1 year)= 1- P(No Root Canal within 1 year) = 1-0.647= 0.353 c. Among patients who attended their first 6-month checkup after enrolling in the study, what was the probability that they would need root canal therapy in the next 6 months? (2pts) IN PREP 0.22 d. Among the 100 individuals in your original cohort, what proportion received root canal therapy? (1pt) 16+15+12= 43/100= 0.43 e. According to your life table results, What was the cumulative probability of root canal therapy for the entire study period (1 pt) IN PREP P(Root Canal for entire study)= 1- P(No Root Canal for entire study) = 1-0.480= 0.52 f. How can you account for these different “probability” estimates? (3pts) The life table took into account people who dropped out. g. Why does it make sense to count those who withdrew as contributing to the denominator by ½ during the time period where they were “lost” in this example? (2pts) We do not know when they were “lost”, so we take an estimate. We assume that all the people dropped out at the beginning of the study. h. Suppose another dental practice performed a similar study at the same time and found the cumulative probability of root canal therapy following a normal molar filling after 1 year to be 15%. Does risk appear to be associated with the choice of dental practice? Make a quantitative comparison as part of your answer. (4pts) P(Root Canal for 1 year)= 1- P(No Root Canal for 1 year) = 1-0.647= 0.353 or 35.3% RR=15/35.3 = 0.425 Yes, the risk is associated with the choice of dental practice. PubH 6003_U: Principles and Practice of Epidemiology Exercise 4

Chew sugarless gum, floss regularly, and taken antibiotics in previous 6 months appear to be protective exposure. While have gum disease, overweight/obese, and recent viral infection appears to be a harmful exposure. c. What doesn’t appear to have any association? (1pt) There no exposure in that list that appears to have no association. d. If the controls are a random selection of anyone who did not have a filling in a molar vs. those who did, how might that impact the ORs? (2pts) If there are other factors related to not having fillings in molars that are also associated with the outcome, this could introduce bias or confounding. It might falsely elevate or diminish the association between fillings and the outcome. e. If ALL participants were patients who also had a molar filling is the OR a good estimate of the RR? Why or why not? (2 pts) The OR is not the same as the relative risk but it is similar when the case is rare. f. Discuss both 1 strength and 1 weakness of using a case-control here (assume they used anyone with a root canal in this practice the last 18 month as cases and any patient from this practice who did not as controls and controls are the same ages as cases) (3 pts) Root canals might be relatively rare events, and a case-control study allows for the focused identification of cases and controls within a confined sample, making it easier to study and analyze these infrequent occurrences. However, there's also a weakness associated with this study design: Using patients from the same practice might introduce selection bias and limit the generalizability of the findings. Patients in the same practice may share similar socioeconomic backgrounds, access to dental care, or other factors that could affect the results. This could limit the ability to generalize findings to a broader population with different demographics or healthcare settings. PubH 6003_U: Principles and Practice of Epidemiology Exercise 4