1. Cross-sectional studies (also known as cross-sectional analyses, transversal studies, prevalence study) form a class of research methods that involve observation of all of a population, or a representative subset, at one specific point in time. They differ from case-control studies in that they aim to provide data on the entire population under study, whereas case-control studies typically include only individuals with a specific characteristic, with a sample, often a tiny minority, of the rest of the population. Cross-sectional studies are descriptive studies (neither longitudinal nor experimental). Unlike case-control studies, they can be used to describe, not only the Odds ratio, but also absolute risks and relative risks from prevalences (sometimes called prevalence risk ratio, or PRR).[1][2] They may be used to describe some feature of the population, such as prevalence of an illness, or they may support inferences of cause and effect. Longitudinal studies differ from both in making a series of observations more than once on members of the study population over a period of time.
Cross-sectional studies in medicine
Cross-sectional studies involve data collected at a defined time. They are often used to assess the prevalence of acute or chronic conditions, or to answer questions about the causes of disease or the results of medical intervention. They may also be described as censuses. Cross-sectional studies may involve special data collection, including questions about the past, but they often rely on data originally collected for other purposes. They are moderately expensive, and are not suitable for the study of rare diseases. Difficulty in recalling past events may also contribute bias. 2. Quasi-Experimental Study
A quasi-experimental study is a type of evaluation which aims to determine whether a program or intervention has the intended effect on a study’s participants. Quasi-experimental studies take on many forms, but may best be defined as lacking key components of a true experiment. While a true experiment includes (1) pre-post test design, (2) a treatment group and a control group, and (3) random assignment of study participants, quasi-experimental studies lack one or more of these design elements.
Since the most common form of a quasi-experimental study includes a pre-post test design with both a treatment group and a control group, quasi-experimental studies are often an impact evaluation that assigns members to the treatment group and control group by a method other than random assignment. Because of the danger that the treatment and control group may differ at the outset, researchers conducting quasi-experimental studies attempt to address this in a number of other ways (e.g., by matching treatment groups to like control groups or by controlling for these differences in analyses). This section focuses on two forms of quasi-experimental studies: a pre-post test design study without a control group and a pre-post test design with a control group. 3. Case Control Studies are studies in which patients who already have a specific condition are compared with people who do not. They often rely on medical records and patient recall for data collection. These types of studies are often less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean than one factor necessarily caused the other. 4. Stability Study
Definition
A quality notification of the notification type QS or QR, which serves as the central object for the stability testing of physical samples.
Use
Business View
A stability study documents a request to examine and test a substance or product over a specified period of time, to examine the effects of different storage conditions on the characteristics of this substance or product. A stability study is usually conducted over an extended period of time to determine the long-term effects of these conditions.
Technical View
The stability study is represented in the system with the help of a quality notification. The quality notification is the business object that is used to process and manage all of the activities that relate to a stability study. You use the standard functions for creating and processing quality notifications to create and process stability studies.
The system provides the following quality notification types for use in stability studies: * Notification type QS (for stability studies conducted with reference to a material) * Notification type QR (for stability studies conducted without reference to a material)
These quality notification types have all of the necessary settings for creating and processing a stability study in the standard manner described in this documentation. If necessary, however, you can customize the quality notification type and the functions of the quality notification's action box to adapt the process to your company's specific requirements.
You trigger the different steps in the stability study process using a selection of predefined follow-up functions in the quality notification's. These follow-up functions are sorted in a logical sequence, to allow you to perform all process steps in the proper sequence from a single point.
Integration
When you execute various process steps in a stability study, the system can automatically assign the following additional objects to the study, depending on which steps are performed: * Initial sample * Documents * Stability bill-of-material (only possible with quality notification type QS - stability study with reference to a material) * Inspection lots for initial test and stability tests * Stability samples for storage conditions * Testing schedule
5.Case series and Case reports consist of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.
Experimental studies
• Clinical trials
• Pharmaceutical research
• Educational assessments
Clinical trials are sets of tests in medical research and drug development that generate safety and efficacy data (or more specifically, information about adverse drug reactions and adverse effects of other treatments) for health interventions (e.g., drugs, diagnostics, devices, therapy protocols). They're conducted only after satisfactory information has been gathered on the quality of the nonclinical safety, and health authority/ethics committee approval is granted in the country where approval of the drug or device is sought.
Depending on the type of product and the stage of its development, investigators initially enroll volunteers and/or patients into small pilot studies, and subsequently conduct larger scale studies in patients that often compare the new product with others already approved for the affliction of interest. As positive safety and efficacy data are gathered, the number of patients is typically increased. Clinical trials can vary in size, and can involve a single research entity in one country or many such entities in multiple countries.
A full series of trials may incur sizable costs, and the burden of paying for all the necessary people and services is usually borne by the sponsor, which may be a governmental organization or a pharmaceutical or biotechnology company. When the diversity of required support roles exceeds the resources of the sponsor, a clinical trial is managed by an outsourced partner, such as a contract research organization or a clinical trials unit in the academic sector.
Randomized controlled clinical trials are carefully planned projects that study the effect of a therapy on real patients. They include methodologies that reduce the potential for bias (randomization and blinding) and that allow for comparison between intervention groups and control groups (no intervention).
Studies that show the efficacy of a diagnostic test are called prospective, blind comparison to a gold standard study . This is a controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study group.

Observational studies
• Cross-sectional
• Case control
• Cohort
Cohort Studies take a large population and follow patients who have a specific condition or receive a particular treatment over time and compare them with another group that has not been affected by the condition or treatment being studied. Cohort studies are observational and not as reliable as randomized controlled studies, since the two groups may differ in ways other than in the variable under study.
8. A Meta-analysis will thoroughly examine a number of valid studies on a topic and combine the results using accepted statistical methodology as if they were from one large study. Some clinicians put Meta-analysis at the top of the pyramid because part of the methodology includes critical appraisal of the selected RCTs for analysis.
9. Postmarketing surveillance (PMS) is the practice of monitoring the safety of a pharmaceutical drug or device after it has been released on the market and is an important part of the science of pharmacovigilance. Since drugs are approved on the basis of clinical trials which involve relatively small numbers of people who have been selected for this purpose - meaning that they normally do not have other medical conditions which may exist in the general population - and postmarketing surveillance can further refine, or confirm or deny, the safety of a drug after it is used in the general population by large numbers of people who have a wide variety of medical conditions.
Postmarketing surveillance uses a number of approaches to monitor the safety of licensed drugs, including spontaneous reporting databases, prescription event monitoring, electronic health records, patient registries and record linkage between health databases.[1] These data are reviewed to highlight potential safety concerns in a process known as data mining.
10. Systematic Reviews usually focus on a clinical topic and answer a specific question. An extensive literature search is conducted to identify all studies with sound methodology. The studies are reviewed, assessed, and the results summarized according to the predetermined criteria of the review question. The Cochrane Collaboration has done a lot of work in the area of systematic reviews.