The so-called cohort study refers to

Cohort study is an observational research method, which divides specific people into different subgroups according to whether they are exposed to a suspicious factor or exposure degree, tracks and observes the outcomes of two or more groups of members (such as diseases), and compares the differences in the incidence of outcomes among groups to determine whether there is causal relationship and correlation between these factors and outcomes.

Observational research is a research method to collect data by observing and recording phenomena, which is often used to study human behavior, environment and health. Observation research includes three main types:

1. Cross-sectional study: Cross-sectional study, also known as "cross-sectional survey", refers to a survey of a group at the same time. Cross-sectional study aims to understand the relationship between different variables at a certain point, such as the relationship between a disease and factors such as age, gender and living habits.

2. Case-control study: Case-control study is a retrospective study, which is usually used to study rare diseases or diseases that need long-term follow-up. Case-control study determines the risk factors that may lead to a certain disease by comparing the differences between the patients (case group) and the patients (control group).

3. Cohort study: Cohort study is an observational study, which is usually used to evaluate the relationship between exposure and health outcomes. Cohort study determines the relationship between exposure and results by selecting a group (cohort), recording their specific factors at the beginning, and then tracking these people during follow-up. Cohort study can be prospective, that is, determine the exposure and results of participants before data collection begins; It can also be retrospective, that is, using historical data to determine exposure and results.

The limitations of observational studies include the following aspects:

1, causality can't be proved: observational research can't control variables through experiments, so it's impossible to determine whether one factor is the cause of another (that is, causality can't be proved), and only the correlation between them can be found.

2. Affected by various interference factors: In observation and research, it is difficult to exclude the influence of other interference factors on the results. For example, there may be other unrecorded factors in the population that may interfere with the observation results.

3. Memory bias: In retrospective study, subjects may have memory bias, which makes them unable to accurately recall past behaviors or events.

4. Sample selection bias is difficult to control: for observational research, sample selection is a very important step, but it is also prone to selection bias. For example, if the sample is not representative of the population, then the results are not universally applicable.

5. There may be information bias: Because there is no control group and experimental group in the observational study, participants and investigators will know the treatment methods or exposures they have received, which may lead to information bias.