Read this article first;
https://acupunctureherbalmedicine.blogspot.com/2023/05/the-type-of-studies.html
'Retrospective and Prospective study' / 'Cohort study and Case control study'
Prospective Studies: These studies are designed before the outcome of interest (like a disease or other health event) has occurred. Prospective studies follow subjects over time and monitor for the outcome. When the study begins, researchers record the exposure status (such as a certain behavior or treatment) of all participants. The researchers then track the participants forward in time to see if and when they develop the outcome. This allows the researchers to directly observe how the exposure might influence the risk of the outcome.
Retrospective Studies: These studies start after the outcome has already occurred. The researchers look back in time (hence "retro"-spective) to gather data on the exposure status of each participant. This might involve reviewing medical records, conducting interviews, or using other data sources that provide information about the participants' past.
Cohort Studies: Cohort studies can be either prospective or retrospective. They follow a group (cohort) of individuals over time, tracking exposures and outcomes. In a prospective cohort study, you identify your cohort and measure their exposures (like smoking or diet), and then follow them forward in time to see who develops the outcome (like lung cancer). In a retrospective cohort study, you identify a cohort based on past data and determine both their exposures and outcomes from past records.
Let's take lung cancer and smoking. You could start with a group of people who are currently smokers and a group of people who are non-smokers. None of these people have lung cancer at the start of the study. You then follow these groups over 20 years, for instance, checking periodically to see who develops lung cancer. After 20 years, you might find that a higher proportion of the smokers have developed lung cancer compared to the non-smokers, suggesting an association between smoking and lung cancer.
Case-Control Studies: These are always retrospective. They start with people who already have the disease (cases) and people who do not (controls). The researcher then looks back in time to see what exposures might be different between the two groups. The purpose is to identify factors that might have contributed to the development of the disease in the cases.
For lung cancer and smoking, you'd start with a group of people who have been diagnosed with lung cancer (cases) and a group of similar people who do not have lung cancer (controls). You then use medical records or interviews to determine who was a smoker in the past. If a higher proportion of the lung cancer group were smokers compared to the control group, this suggests an association between smoking and lung cancer.
Comparing Cohort and Case-Control Studies:
In cohort studies, the researcher follows a group of people over time to see who develops the outcome. This allows them to directly measure incidence (new cases) of the disease and calculate relative risks. Because they start before the outcome has occurred, they can also more accurately measure exposures. But these studies can be time-consuming and expensive, and may require a large number of participants to have enough cases of the outcome to study.
Case-control studies, on the other hand, are generally quicker and less expensive than cohort studies, because they start with people who already have the disease. They're particularly useful for studying rare diseases or diseases with a long latency period. However, since they start after the outcome has occurred, they may be more prone to bias in measuring exposures (recollection bias, for example), and they can only calculate odds ratios, not relative risks.
A key difference is that cohort studies can examine multiple outcomes (as they track participants over time and see what happens), whereas case-control studies are usually focused on a single outcome (the disease that defines the cases).
Examples
1. Prospective Cohort Study:
- Cohort study: DOI: 10.1177/15347354221123055
2. Retrospective Cohort Study:
- Retrospective cohort study: DOI: 10.1111/1471-0528.15925
Suppose you have access to medical records from a large healthcare system that goes back 30 years. From these records, you could identify two groups of people: those who were smokers 30 years ago, and those who were not smokers at that time. Importantly, none of these people had lung cancer at the start of this 30-year period. Then, using these same medical records, you could determine how many people in each group have been diagnosed with lung cancer over the past 30 years. You don't need to follow them forward in time because the data has already been collected; you're simply analyzing it in retrospect.
If a higher proportion of the past smokers developed lung cancer compared to the non-smokers, it would suggest that smoking is associated with an increased risk of lung cancer. This is the same conclusion you might draw from the prospective cohort study, but the retrospective study could potentially be completed much faster, since you're working with already-existing data. However, the accuracy of the results depends on the quality and completeness of the past records, which could be a limitation.
Now, let's use the herbal medicine and diabetes example. Suppose you have access to health records from National insurance in Korea for 30 years. You could identify a group of people who took herbal medicine on regular basis and a group who didn't. Then, using medical records, you could determine how many people in each group have developed diabetes over those 30 years. If fewer people in the herbal medicine group have developed diabetes, it might suggest that taking herbal medicine is associated with a lower risk of diabetes.
3. Case-Control Study:
For the Tai Chi and diabetes, you'd start with a group of people with diabetes (cases) and a group without diabetes (controls). You then look back to see who had a regular Tai Chi practice. If a larger percentage of the control group (those without diabetes) were practicing Tai Chi compared to the cases (those with diabetes), this would suggest that Tai Chi might be protective against diabetes.
- Case-control study: DOI: 10.1016/j.jep.2021.114291
In a nutshell;
If the study started from cause of disease, it is cohort.
If the study started from disease or not, it is case control study.
Cohort and Case-control study in TCM
These types of studies are typically employed in epidemiological research rather than clinical efficacy studies. Hence, applying cohort and case-control studies to fields like acupuncture or herbal medicine is not common. This is largely due to the challenges in defining exposure in these observational studies. Unlike clear-cut categorizations such as smokers vs non-smokers, it's less straightforward to classify individuals into acupuncture users and non-users. Variables like the type, frequency, and duration of acupuncture treatments can greatly vary among users, which makes it challenging to establish clear exposure groups for such observational studies.
Retrospective Cohort study in TCM
Implementing retrospective cohort studies under normal circumstances can be challenging as they require comprehensive data from all individuals within the interested population. These studies are primarily achievable in settings with a high degree of control or extensive coverage, such as military bases, isolated ships or islands, or countries like Korea and Taiwan where the government provides universal health insurance, including for acupuncture. Consequently, this unique type of study is relatively rare in TCM fields but is frequently found in Korean and Taiwanese research. This is because it requires minimal resources and is not time-consuming, despite being labeled as a 'cohort' study. It's important to understand that a retrospective cohort study does not offer evidence as strong as a true prospective cohort study, despite its name. According to Dr. Sackett, the evidence derived from a retrospective cohort study is typically considered as weak as that from a case-control study.