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. ِ

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 an inference.

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

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

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.

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).

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).

3. Cross-sectional: (prevalence study, photograph, snap shot, … 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.

Disadvantages: - Risk cannot be measured 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.

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

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

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:

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

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

- Problems of control group:. No. should be 1-4 times . - 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.

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

Prospective: (longitudinal, incidence or follow up, 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 study exposure 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 study outcome

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.

Disadvantages: - Time, efforts & money consuming 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.

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.

Types: Therapeutic trial: - Two different drugs or procedures 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.

Types Randomized controlled trials (clinical trials) Cluster randomized controlled trials Field trials Community trials

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.

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