Observational studies. We observe one or more groups of subjects and their characteristics are described and/or analyzed. Experimental. We can manipulate variables. Series of cases. To describe characteristics of a patients group. Leads to the generation of hypothesis; use a short time period and there is not a control group. Cases and controls. The beginning is with the presence or absence of outcome y review before in the time to detect risk factors. Cohort. The beginning is with a subjects group with outcome; then, we classified them in exposed or non-exposed, and follow in the time to see if they develop the outcome. Cross-sectional. We measure the exposure and the outcome in a subjects group simultaneously, in a point of the time. Experimental. The beginning is with a cohort and randomize it to receive the intervention (experimental group) or not receive it (control group); then we follow to the subjects in the time to see if develop the outcome of interest. Ecologic. The main characteristic is that study groups not individuals.
They are called retrospective because of their direction in the time. Sometimes are matched cases with controls to make them more similar.
It begin as cohort study, classify the subjects as exposed or non-exposed; in the follow up, some participants develop the outcome, and they are the cases and a sample of subjects without outcome are controls and with them analyzed the cases-controls study. An important advantage is that cases and controls are members of the same population, because the probability of selection bias is minimized. Another advantage is that the information about the exposure is collected before of the develop of outcome, and the probability of information bias is minimized.
A cohort is a subjects group or things that they have some in common and do not have the outcome of interest.
They are called prevalence studies. They help us to know the economic, social burden of a disease in a population in a point of time. They can be: descriptive or analytic. The descriptive only show the frequency of exposure and of outcome in population in a point of time; in analytic studies we search to show the exposure and outcome in a point of time, searching associations with Prevalence Ratio or Odds Ratio.
También se les llama pruebas clínica o estudios intervencionales.
Types of studies Types of epidemiological studies Experimental Observational Randomized Quasi- Controlled Experi- studies mentalCases or report Cases and Cohort Cross sectional Ecologic of cases controls
Cases in series Advantages: They are easy to write. The observations are useful to other researchers. Disadvantages: There is a lot of bias.
Cases and controls studies Exposed Cases Non-exposed Exposed Controls Non-exposedDirection of research Beginning of study Time
Cases and controls studies Advantages: They are adequate to study rare outcomes. They are adequate to outcomes with long latency period. They are cheap and easy to apply. It is not necessary to wait to present outcome. Disadvantages: A lot of bias. They depend on the quality of registries. Control group should be adequately selected, because they represent the population without the outcome.
Cohort studies With outcome Cases Exposed Without outcomeCohort selected Sample ofto study controls With outcome Cases Without Non- outcome Sample of exposed controls Beginning of Time study
Cohort studies Exposed With outcomeSelection of a Withoutcohort for study outcome With outcome Without Non-exposed outcome Beginning of Time study
Cohort studies Subjects are selected because do not have the outcome and they are classified if have or not have the risk factor (exposure). We follow up to prove if they develop the outcome. The cohort study can be prospective if the follow up is forward in the time or it can be retrospective (historic), if it go back in the time.
Cohort studies Advantages: They are adequate to know the causes of an outcome. To know the natural history of disease. They adequate when the exposure is rare. They are useful when we study two or more outcome at the same time. Disadvantages: They take a long time. They are expensive. Subjects can be lost in the follow up. They are not adequate for study rare outcomes.
Cross sectional studies Exposed with outcome Subjects selected to study Exposed without outcome Non-exposed with outcome Non-exposed without outcome Beginning of study
Cross sectional studies Analyze data of a subjects group in a point of time. Describe a disease and its importance for the population. Define the needed on health. They can be classified in: Descriptive Analytic
Cross-sectional studies Advantages: They are useful to know the burden of a disease in a group. Useful to evaluate diagnostic procedures. To study common risk factors. To study common outcomes. Disadvantages: Populations little willing to collaborate. The sample can not be representative from the population. It is not useful to search causes of the outcome.
Experimental studies Classification Randomized clinical trials. Quasi experimental. With historic controls.
Experimental studies Exposed Outcome WithoutSubjects that outcomeparticipate Outcome Controls Without outcomeBeginning of study Intervention Time
Experimental studies They are called clinical trials. It is administrated an intervention to a group, randomize selected and we do not know what is receiving (blind). The group that does not receive the intervention, it is a control group. The allocation of subjects in experimental or control group is given by chance. By ethics reasons, only it is permitted beneficial interventions.
Experimental studies Blind single is when the subjects do not know what intervention are receiving. Double blind is when neither subjects nor researcher know what intervention are receiving each subject.
Experimental studies There are clinical trials with auto controls. The same group work as control group.
Experimental studies There are cross design where it is administrated an intervention (1) to experimental group and another (2) in a control group. After, interventions are suspended, and left a space (wash out period) without it, then the intervention 1 is administrated to control group and intervention 2 is administrated to experimental group.
Experimental studies Outcome Outcome Experimental group ControlsSubjects thatparticipate Without Without outcome outcome Outcome Outcome Experimental Controls Without group outcome Without outcomeBeginning of Intervention Intervention Timestudy
Experimental studies There are clinical trials with external controls. We compare the results with the results of another researcher or with the results of a previous study. Also, they are called historic controls.
Experimental studies With outcome Subjects Without outcome With outcome Results of a previous study Without outcomeBeginning Intervention only in subjects Timeof study
Experimental studies Advantages: Give evidence strong of causality. There are less bias. Historic controls are used in preliminary studies. Disadvantages: Inappropriate use of historic controls lead a severe mistakes. Expensive. They need time.
Ecologic studies Compare exposure and the outcome between groups. Measure the exposure and outcome, in the group as all. They are only studies that offer to study differences between groups.
Ecologic studies Advantages: Fast Cheap Use routinely data Disadvantages: They did not take into account to the individual. They depend on the quality of routinely data They are difficult to interpret.
Bibliography 1.- Gordis L. Epidemiology. Phialdelphia, W.B. Saunders Company, 1996. 2.- Songer T. Study designs in epidemiologic research. Supercourse, 2005 ( http://www.pitt.edu/~super1/lecture/lec19101/index ) (Accesed October 2008). 3.- Hennekens CH, Buring J, Mayrent SL. Epidemiology in Medicine. Boston, Little Brown and Company, 1987.