Clinical studies are a critical component of evidence-based medicine. They provide the scientific basis for the management and treatment of diseases. Understanding the taxonomy of clinical studies is essential for researchers, clinicians, and anyone involved in healthcare decision-making. This article will delve into the classification of clinical studies based on their design characteristics.
Descriptive Studies
Descriptive studies are the most basic type of clinical study. They do not have a comparison group. Instead, they aim to describe the characteristics of a particular disease or condition, or the distribution of these characteristics in a population. Examples of descriptive studies include case reports, case series, cross-sectional studies, and surveillance studies. These studies are often the first step in identifying a new disease or condition and can provide valuable information about its prevalence and distribution.
A group of clinicians might publish a case series describing the symptoms, treatment, and outcomes of patients who received acupuncture for their headaches. They might report that a significant number of these patients experienced a reduction in headache frequency and severity following acupuncture treatment. However, without a comparison group, it's impossible to say whether the improvement was due to the acupuncture or some other factor.
Analytic Studies
Analytic studies, on the other hand, do have a comparison group. They aim to identify and quantify associations, test hypotheses, and identify causes. Analytic studies can be further divided into experimental and observational studies.
Experimental Studies
In experimental studies, the researcher assigns the comparison group by force. The most common type of experimental study is the randomized controlled trial (RCT). In an RCT, participants are randomly assigned to either the treatment group or the control group. This random assignment helps to ensure that any differences in outcomes between the groups are due to the treatment and not to other factors.
In a randomized controlled trial, researchers might randomly assign patients with headaches to one of two groups: one group receives acupuncture, and the other group (the control group) receives a placebo treatment (like sham acupuncture). The researchers would then compare headache frequency and severity between the two groups. If the acupuncture group shows a significant improvement compared to the control group, this provides strong evidence that acupuncture is an effective treatment for headaches.
Observational Studies
If the comparison group is not assigned by force, the study is an observational analytic study. These studies observe individuals in their natural settings. Within observational studies, we can further distinguish between cohort studies and case-control studies.
Cohort Studies (Exposure-to-disease observational analytic study)
Cohort studies start the analysis from the cause. They follow a group of people (a cohort) over time to see how their exposures affect their outcomes. Cohort studies can be prospective (the cohort is followed forward in time) or retrospective (the cohort is identified from past records and followed from that time forward).
In a cohort study, researchers might identify two groups of people: one group who regularly consumes herbal tea (the exposed group) and another group who does not consume herbal tea (the unexposed group). Both groups should be similar in other aspects, such as age, gender, and overall health status, to ensure that the effect of herbal tea consumption can be isolated.
The researchers would then follow these two cohorts over a certain period, regularly checking their blood pressure levels and recording any instances of hypertension. If the incidence of hypertension is significantly lower in the herbal tea-consuming group compared to the non-tea consuming group, this could suggest that herbal tea consumption may be associated with a lower risk of hypertension.
In a cohort study, researchers might identify a group of patients who choose to receive acupuncture for their headaches and a similar group who choose not to receive acupuncture. They would then follow these two cohorts over time and compare the incidence of headaches. If the acupuncture cohort has a lower incidence of headaches, this suggests that acupuncture may be an effective treatment. Acupuncture cannot be a good resource for cohort study, because it is hard to define acupuncture as an meaningful exposure. Relatively, retrospective cohort study is common in acupuncture study.
Case-Control Studies (Disease-to-exposure observational analytic study)
Case-control studies, on the other hand, start the analysis from the effect. They begin by identifying individuals with the outcome of interest (cases) and individuals without the outcome (controls). The researchers then look back in time to see what exposures might have led to the outcome. Case-control studies are particularly useful when the outcome is rare or when it takes a long time to develop.
In a case-control study, researchers would start by identifying individuals who have hypertension (the cases) and individuals who do not have hypertension (the controls). The controls should be similar to the cases in terms of age, gender, and other relevant factors, except for the presence of hypertension.
The researchers would then look back in time and ask both cases and controls about their past herbal tea consumption habits. If a significantly higher proportion of controls (those without hypertension) report regular herbal tea consumption compared to the cases (those with hypertension), this could suggest that herbal tea consumption may be associated with a lower risk of developing hypertension.
In a case-control study, researchers might start by identifying individuals who experience frequent headaches (the cases) and individuals who do not (the controls). They would then look back in time to see which individuals had received acupuncture. If a higher proportion of controls than cases had received acupuncture, this suggests that acupuncture may be protective against headaches.
Remember, while both study designs can suggest an association between herbal tea consumption and hypertension, or acupuncture and headache treatment, they cannot definitively establish a causal relationship due to potential confounding factors and biases. Randomized controlled trials are needed to confirm these findings and establish causality.
Telling Study Types with simple questions
Does it have a comparison group?
This is the first question to ask when classifying a clinical study. A comparison group is a set of individuals who are similar to the group receiving the treatment or intervention but do not receive the treatment or intervention themselves. They serve as a benchmark to compare the effects of the treatment or intervention.
- Yes - Analytic Study: If the study has a comparison group, it's an analytic study. Analytic studies aim to evaluate the relationship between exposures (like interventions or risk factors) and outcomes, and they do this by comparing groups.
- No - Descriptive Study: If the study does not have a comparison group, it's a descriptive study. Descriptive studies aim to describe the characteristics of a group or population, or the nature of a condition, without drawing conclusions about cause and effect.
Is the comparison group assigned by force?
If the study is an analytic study, the next question to ask is whether the comparison group is assigned by force, meaning the researchers control who receives the treatment or intervention.
- Yes - RCT/Experimental Study: If the comparison group is assigned by force, it's a randomized controlled trial (RCT) or other type of experimental study. In these studies, participants are randomly assigned to receive the treatment or intervention or to be in the control group.
- No - Observational Analytic Study: If the comparison group is not assigned by force, it's an observational analytic study. In these studies, researchers observe participants in their natural settings without intervening.
Did an observational analytic study start analysis from cause(exposure)?
If the study is an observational analytic study, the next question to ask is whether the study starts the analysis from the cause, meaning it begins by identifying a group with a particular exposure or treatment and follows them over time.
- Yes - Cohort Study: If the study starts the analysis from the cause, it's a cohort study. Cohort studies follow a group of people over time to see how their exposures affect their outcomes.
- No - Case-Control Study: If the study does not start the analysis from the cause, it's a case-control study. Case-control studies begin by identifying individuals with the outcome of interest (cases) and individuals without the outcome (controls), and then look back in time to see what exposures might have led to the outcome.
Extra resources
 |
| https://www.cdc.gov/globalhealth/healthprotection/fetp/training_modules/19/ |
 |
| https://www.cisncancer.org/research/how_cancer_is_studied/epidemiological/study_types.html |