HS 2801 Notes on Cohort Studies PDF

Week 7 Cohort studies - Formation of a cohort, which is a group of individuals followed over time 1. Those who enter cohort should be outcome free 2. Selecting exposed and comparison groups 3. Following up participa...

Week 7 Cohort studies - Formation of a cohort, which is a group of individuals followed over time 1. Those who enter cohort should be outcome free 2. Selecting exposed and comparison groups 3. Following up participant 4. Determining outcome status - Exposure is determined before the outcome happens - More expensive, time consuming - Not efficient for diseases with long latent periods - Better exposure and confounder data - Less vulnerable to information bias Prospective: starting the study with the cohort and looking at results Retrospective: starting study with the incidence of disease - Cheaper, faster, - Efficient with diseases with long latent period - Exposure and confounder data may be inadequate - More vulnerable to bias - Need established recording system Relative risk in cohort studies - Looks at incidence rate ratio - Comparison of the incidence of a characteristic in two independent population that is calculated by taking a ratio of their incidence rates - IR= (a/a+b)/(c/c+d), iR=2.54, probability increased by 2.5 times, or 254% Time of measurement of outcome is crucial feature of cohort studies - Concept of person time: people are in the study for varied durations - Different times under exposure - Contribute differentially to study Cohort studies advantages - Valuable when exposure is rare - Can examine multiple effect of a single exposure - Easier to determine the temporal relationship between exposure and outcome - Allows measurement of incidence Cohort studies disadvantages - Validity affected by losses to follow up (selection bias) - Other factors may not be distributed evenly between exposure group (confounding) - Inefficient for evaluation of rare disease - Can be expensive and time consuming (need large numbers and long follow up_ - If retrospective they require good records Observation studies - case control studies - If needs are not met with cohort do case control - Large numbers of subject followed for long periods of time = difficult, time consuming, expensive - Loss to follow has potential to undermine validity - Not good for rare diseases with long latency periods - Not good when exposure data is expensive to obtain - Working backwards Case control data - Can not calculate measure of occurrence: risk and rates - No long have denominator as entire population at risk (controls only represent a sample of population with arbitrary size) - No idea about real prevalence in exposed or unexposed, hence no information on risk - Can only calculate odds ratio = a/b / c/d Selection of cases - Very clear case definition required - Who are the cases of disease in study - Ideally case selection will involve direct sampling within a source population - All people in source population who develop disease of interest will be included as cases - If someone in source population develop disease of interest, would they meet the case definition and be included as a case in the study Control: a sample of the source population that produced the cases Purpose: to estimate the exposure distribution in the source population that produced the cases - Knowing the exposure prevalence among cases and control is what allows us to measure the association between exposure and outcome - Selection of controls - Must come from the same source population as cases - Random selection is necessary to obtain a representative sample of source population - Representativeness is very important consideration - Control must be selected independently of exposure - Means that their exposure status does not influence selection - Where to find controls - Population based controls (random selection) - Controls selected from general population, most suitable when cases are from well defined area - Cell phone or internet subscribers, residence lists, tax lists - Time consuming, hard to inspire participation, may not recall past exposures well - Controls often come from well defined source population - Nested controls from cohort population - Controls selected from an existing cohort population - Controls represent a sub set of full source population - Controls come from clearly defined source population, already enrolled→ willing participants - Hospital or clinic based control - Controls selected among patients at hospital or clinic - Choose controls with disease other than the case disease - Typically used when cases are identified from a hospital - Illnesses that have same catchment area as cases (same source population) - Illness that have no known relation to risk factor under study (selection of control should be independent of exposure)  - Easy to identify and access, less time and money, accuracy of exposure recall comparable to cases, willing to participate - These controls are not randomly selected, hospital based controls must be carefully selected to accurately represent the exposure history in source population - Family or friend control Definition/purpose - Dictionary of epidemiology: the process of making a study group and comparison group comparable with respect to extraneous factors - Makes groups similar as possible to account for confounding - Randomization like in experimental studies - Only difference is that exposure status (intervention) making exposure groups similar in other factors - Matching cases to controls - Only difference; outcome status, making outcome groups similar in other factors - After selection of matching factors, for each case a control with the same characteristics will be selected - Characteristic with the highest possibility of being a confounder (age, sex, setting) - Like randomization in experimental studies the only difference remains in - Exposure status; making exposure groups similar in other factors to account for confounding - Analysis - Unit is not person but a pair (case and matched control) - Paired analysis - Using pairs in the table - Types of matching in case control - Individual matching (performed participant by participant - Frequency matching (providing similar distribution of confounders in groups) - Matching in cohort studies - Exposed matched to unexposed to mimic randomization - Less common and less - Expensive - Unpractical sometimes - May require control (competing risks, loss to F/U) Case Control studies advantages and disadvantages - Advantages: - More efficient than cohort study (time, money, effort) - Suited to disease with long latent period - Optimal for rare disease - Can examine multiple exposures - Disadvantages - Exposure is assessed after development of outcome (maybe unsure about temporal sequence between exposure and disease/recall bias) - Prone to selection bias in control of choice - Usually only study one disease or outcome - Inefficient for rare exposure - Can not calculate absolute measure of association