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Study design

Learn about the different types of study designs commonly used in medical research, their characteristics, advantages, and limitations, and how to choose the right design for a strong and valid study.

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Study design

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  1. Study design

  2. ِ Objectives: • To know the different types and varieties of designs that are commonly used in medical researches. • To know the characteristics, advantages and limitations of each type in order to know how to choose a proper design that helps conduct a strong, efficient and valid study that is neat of errors and bias.

  3. Types of studies A study is an organized, systematic, scientific search for the truth in the community. Study design: Is the main structure of the study that determine it’s approach, plan, and steps of conduction and implementation to reach the inference.

  4. We design a study according to: • Objective(s) od the study • Nature of the research question • Our resources • The inference we aim or expect to reach

  5. We have two main types of epidemiologic studies: 1. Descriptive II. Analytic

  6. I. Descriptive studies Characterize the occurrence of disease in terms of: person-place-time, or host-agent-environment, characterizes exposure and susceptibility state. It enables us to develop (formulate) hypotheses about the disease pattern and also to identify the high risk population.

  7. 1.Case report: describes clinical observations, interesting or unusual variation of a disease.Advantages: - Easy, simple, quick, inexpensive.- Calls attention to unexpected findings.Disadvantages: cannot be generalized (single case and not population based).

  8. 2. Case series:Refers to a group of similar cases which may enable us to discern a clinical pattern to identify characteristics common to the cases.Advantages:-Easy, simple, quick, and inexpensive.- Recognition of a new disease or beginning of an epidemic.Disadvantages:- Cannot be generalized (small sample size).- Estimation of risk cannot be done (because of lack of a control group).

  9. 3. Cross-sectional:(prevalence study, photograph, snap shot, ….)describe or identify health problems in the entire population (or a sample of the population) at a point in time or over a short period of time.Advantages:- Inexpensive & not time consuming.- Measures the prevalence.- Can study many variables at the same time.

  10. Disadvantages:- Risk cannot be measured.- Chicken-egg dilemma; because the exposure & disease are assessed at the same time.- Selecting survival (usually in fatal disease).- Bias: memory, recall, interviewer, loss of accuracy of temporal relationship.

  11. 4. Ecologic (Correlation) studies Measures that represent characteristics of entire population are used to describe disease and to postulate causal association. Strengths: Cheap, quick and simple (generally make use of secondary data). Limitations: • Can not link exposure-disease relationship at the individual level • Uses average exposure levels rather than actual levels of exposure • Inability to control for confounding factors

  12. 5. Biometry 6. Hospital-records study 7. Meta analysis (Generally descriptive studies are used only to suggest hypotheses & not to test them.)

  13. II-Analytic-studies:go beyond simply describing the distribution of occurrence of diseases & attempt to analyze the reasons for them (testing etiological hypotheses).So hypotheses derived from descriptive studies may be tested by analytic studies.Factor & disease can be the dependent or independent variable according to the approach of the study.It is of 2 types:

  14. A- Observational:In which we just observe (then analyze) what is going on without interference- Case control studies- Cohort studies

  15. 1.Case-control:in which subjects are selected on basis of presence or absence of disease.Advantages:- Easy, quick, inexpensive & can support (but not prove) casual hypotheses.- Useful in rare diseases but not rare exposures.- Odds ratio can be estimated from it.Disadvantages:- Cannot measure incidence or RR.- Bias: selection, memory, and recall

  16. .- Problems of control group:* No. should be 1-4 times.* Matching (particular & conceptual problem).* Randomization (problem of specific control).* Types of control group.Nested case control study:A case control study inside the cohort prospective to decrease time, money & efforts, it will identify the risk factors & eliminate re-call bias.

  17. 2. Cohort study:a forward looking study (movie picture) in which subjects are selected on bases of presence or absence of exposure. (The results are expressed in incidence).Types:- Retrospective cohort: investigate persons known to have a disease before the time of the study, but the problem is that we cannot know which precedes which: the factor or the disease. (So RR cannot be estimated). It is especially important in diseases with long latency period.Exposure out come * start of the study

  18. Prospective:(longitudinal, incidence or follow up,..): study of persons free of the disease at the start of the observation period (the most common type of cohort).* Start of the studyexposure outcome - Historical prospective: similar to the prospective but some of the persons have been already exposed before the start of the study by a certain period & then continue till reach the defined period of the study.Exposure* Start of the studyoutcome

  19. Advantages:- Test hypotheses of casual relationship & give directly RR.- Temporal sequence between exposure & disease can be more clearly established.- Confirm causes, and magnitude of risk can be quantified more clearly because it gives us incidence rates.- Particularly suited for assessing the effects of rare exposures.

  20. Disadvantages:- Time, efforts & money consuming.- Problems of non-response or drop out (attrition).- Not feasible for disease of low incidence because non or very few persons in the sample may actually manifest the disease.

  21. B- Interventional: (randomized trial)A high quality type of cohort studies in which the investigator himself allocates the exposure, means we don’t only observe but do interfere in the form of a new element which could be a new drug, surgical procedure, vaccine, test, or new method of diagnosis.So it has the potential to provide a degree of assurance about the validity of a result that is not possible with any observational design option.

  22. Types:Therapeutic trial:- Two different drugs or procedures.- Drug & placebo- Single drug (before & after Rx).Preventive trial:- Individuals (as in polio vaccine).- Entire pop. (community trial) an in floride fortification of water.

  23. Types • Randomized controlled trials (clinical trials) • Cluster randomized controlled trials • Field trials • Community trials

  24. Unique problems:- Ethical: we must be sure that it is not harmful, never the less we must inform them about the aim of the study & the possible complications, get their acceptance & decide when to stop & when to proceed, there must be sufficient doubt about the agent to be tested to allow withholding it from half the subjects & at the same time there must be sufficient belief in the agents potential justify exposing the remaining half.- Feasibility: should be considered before a believe gets spread.- Cost.

  25. Other problems:- Chance: managed by increase sample size & randomization.- Bias (subjectivity): treated by masking & blinding (single, double & triple blinding)- Confounders: by matching of all factors (or as much as we can) except the factor under study

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