Cohort Studies Dr. Sameh Zaytoun (MBBch, DPH, DM, FRCP(Manch), DTM&H(UK),Dr.PH) University of Alexandria - Egypt Consultant of Preventive Medicine Al- Hada Armed Forces Hospital

Cohort  Term taken from the ancient Roman term for a group of soldiers that marched together into a battle field.  i.e (a group of subjects followed together over time).

Cohort study Design Design in Words:  Select representative persons with the study exposure  Select comparable Controls without the study exposure  Follow both groups equally to assess who develops the outcome  Compare Risk of developing the outcome in the Exposed and in their Controls

Interpretation of Cohort study  All else being equal, if the Risk of the Outcome differs between the Exposed and the Non-Exposed by more than chance variation, the Exposure is said to be related to the Outcome

Analysis of Ideal Cohort Study Design Exposure Present Exposure Absent Outcome Present Outcome Absent Outcome Present Outcome Absent Outcome + ve Outcome - ve TotalRiskOdds Exposure + veABA + B A / (A+B) A / B Exposure - veCDC + D C / (C +D) C / D Risk Ratio = A / (A+B) C / (C+D) Strength of Association Odds Ratio = A / B C / D = AD / BC

Timing of Cohort study  Concurrent Cohort Study: The cohort is assembled in the present and followed into the future to determine disease incidence  Retrospective Cohort Study: The cohort was assembled in the past and determination of disease incidence is conducted in the present

Concurrent Cohort study Advantages:  Temporal relation between Exposure and Disease can be established reliably  Exposure can be measured accurately not dependent on recall or records  Exposure assessment not Biased by knowledge of the Outcome

Concurrent Cohort study Disadvantages: Expensive and time consuming Can not be used for Rare Diseases

Retrospective Cohort study Advantages:  Same Advantages as concurrent cohort studies  Much less costly and time - consuming

Retrospective Cohort study Disadvantages: Can only be used if data are available about Exposure, Disease, and Confounders at start of the follow up period Investigator has no control over quality of Exposure measurements

Selection of Subjects:  Exclude subjects who are not capable of developing the disease (e.g. women with hysterectomy in a study on uterine carcinoma)  Restrict to subgroups with a fairly high incidence of the disease (e.g hip fracture in the elderly)

Obtaining Exposure information:  Quality of results depends on quality of measurements  Some Exposure variables may change over the follow – up period (e.g. exercise habits), so multiple measurements may be required

Obtaining Disease information:  Outcomes should be assessed using a standardized criteria  Outcomes should be assessed blindly without knowledge of exposure status

Reducing loss to follow – up:  Exclude subjects who will be very difficult to follow  Collect information (e.g. name of subject physician, relative) that will allow subjects to be found if they move or die  Maintain periodic contact during the follow – up period (e.g yearly phone calls, birthday cards)

Difficult -to- Trace Subjects:  Contact the subject physician, relatives  Request forwarding address from the postal service  determine vital status from the National Death Index (in the United Kingdom)  seek address through other sources (e.g. life insurance, voter registration, police offices…..)

Person - Time Concept:  Used to account for person –to- person differences in time under observation  Each person is given credit for just (only) the time being observed  Person.Years = Time in years x persons followed  Time and Persons followed are weighted equally - 10 persons followed 1 year = 1 person followed 10 years = 10 person.years - best when risk of outcome is relatively homogeneous over time and degree of exposure is equal over time.

Cohort points of Vulnerability: Ascertainment of Study Outcome without Bias:  Assessment Bias: - Due to knowledge of Exposure - Increase the publicity about the possible association  Remedies: - Mask assessors from the Exposure status - Use standard protocol to assess / validate the Outcome - Assess more than one Outcome to avoid telegraphing the exact association sought

Comparison: Case - Control Concurrent Cohort Retrospective cohort Study timeshortestlongShort CostLowHighestHigh Rare DiseaseYesNo Recall BiasYesNo Sample SizeSmallLarge Loss to follow upNoYes IncidenceNoYes Relative RiskApprox.Yes Odds ratioYes