Cohort Studies
Cohort studies are prospective in nature.
You suspect that, for example:
• Exposure to a particular chemical, water from a particular source, etc. seems to lead to a particular disease state, or;
• Treatment with a particular procedure or medication seems to lead to a resolution of particular symptoms.
So you set out to examine some representative cases very closely to see if you are correct. As always, it can be exciting to think that you may have noticed something that no one else has noticed, and you want to proceed according to certain rules to insure that you remain honest and prudent.
Here is a simple schematic of the Cohort study:
It is important to note that what makes the Cohort design "prospective" in nature is that you are working from suspected cause to effect (or outcome). It can be a concurrent study, meaning that you start collecting data now; non-current, typical of a chart review or review of other records, or; a combination of the two.
Remember that:
• You can substitute any outcome of interest for Diseased versus No disease (e.g., chronic high blood sugar versus not chronic high blood sugar), and;
• You can substitute Treated versus Not treated for Exposed versus Not exposed.
Data are obtained from groups who have been exposed, or not exposed, to the new technology or factor of interest (eg from databases). No allocation of exposure is made by the researcher. Best for study the effect of predictive risk factors on an outcome.
Advantages:
- ethically safe;
- subjects can be matched;
- can establish timing and directionality of events;
- eligibility criteria and outcome assessments can be standardised;
- administratively easier and cheaper than RCT.
Disadvantages:
- controls may be difficult to identify;
- exposure may be linked to a hidden confounder; blinding is difficult;
- randomisation not present;
- for rare disease, large sample sizes or long follow-up necessary.
Case Study
Case-control studies are retrospective in nature.
You have patients that are sick with a new disease. There are others from the same area that are not infected. You begin to backtrack to try to find the source or common element of the infection.
It can be very exciting to think that we may have noticed something that no one else has noticed. This is where rules (research design) come into play to keep us honest and prudent.
Here is a simple schematic of the Case-control study:
Remember that:
- You can substitute any outcome of interest for Diseased versus No disease (e.g., chronic high blood sugar versus not chronic high blood sugar), and;
- You can substitute Treated versus Not treated for Exposed versus Not exposed.
It is important to note here what is meant by "retrospective". All this means is that you are working from outcome to suspected cause. You can start collecting data, or you can do a chart review, or you can do a combination. What makes it "retrospective" is that you are working from effect to cause, not vice-versa.
Advantages:
- quick and cheap;
- only feasible method for very rare disorders or those with long lag between exposure and outcome;
- fewer subjects needed than cross-sectional studies.
Disadvantages:
- reliance on recall or records to determine exposure status;
- confounders;
- selection of control groups is difficult;
- potential bias: recall, selection.