Study Design
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Study designs Dr. Nadira Abbas AlBaghli
Experimental Studies or Clinical Trials
Controlled trials
are studies in which the experimental drug or procedure is compared with another drug or procedure, sometimes a placebo and sometimes the previously accepted treatment
Randomized controlled trial
provides the strongest evidence for concluding causation; it provides the best insurance that the result was due to the intervention
To reduce the chances that subjects or investigators see what they expect to see, researchers can design doubleblind trials in which neither subjects nor investigators know whether the subject is in the treatment or the control group. When only the subject is unaware, the study is called a blind trial.
Advantages of Clinical Trials
the gold standard/ causality the least number of problems or biases.
Disadvantages&
great expense long duration.
Observational Studies
case–control cross-sectional (including surveys) cohort studies case–series
case–series study
it is the simplest design in which the author describes some interesting observations that occurred for a small number of patients. not include control subjects generally not planned studies do not involve any research hypotheses important descriptive role as a precursor to other studies
Advantages & Disadvantages of Case– Series Studies
They are easy to write, and the observations may be extremely useful to investigators designing a study to evaluate causes or explanations of the observations.
Disadvantage
case–series studies are susceptible to many possible biases related to subject selection and characteristics observed hypothesis-generating and not as conclusive
Case–Control Studies
Case–control studies begin with the absence or presence of an outcome and then look backward in time to try to detect possible causes or risk factors
Advantages of Case–Control Studies
appropriate for studying rare diseases or events the quickest least expensive studies ideal for investigators who need to obtain some preliminary data prior to writing a proposal for a more complete, expensive, and time-consuming study. good choice for someone who needs to complete a clinical research project in a specific amount of time.
disadvantages
Liable to selection/ recall bias they depend completely on highquality existing records. Data availability Not for rare exposure No frequency measure
Cross-sectional studies
analyze data collected on a group of subjects at one time rather than over a period of time. designed to determine "What is happening?" right now. Subjects are selected and information is obtained in a short period of time
Advantages of Cross-Sectional Studies
Feasible Cheap Quick For screening several dis/ exposures, health planning
disadvantage
Temporal ambiguity Not for rare dis, short duration, rapidly fatal
Cohort Studies
A cohort is a group of people who have something in common and who remain part of a group over an extended time. Cohort studies ask the question "What will happen?" and thus, the direction in cohort studies is forward in time
Advantages & Disadvantages of Cohort Studies
No temporal ambiguity No selection/ recall bias Many outcomes.
advantage
Costly Timely Problem of loss to f.u
Systematic review & meta- Analysis
Summaries of evidence on a topic A review addressing a specific research question ( on treatment, Dx, prognosis, or etiology) using explicit methodology of collecting, selecting, and appraising studies and, whenever appropriate, synthesizing their results quantatively. Meta-analysis: The statistical (quantitative) combination of results from 2 or more studies addressing the same research question.
Determining Q type
Therapy: the effect of different therapy Harm: the effect of potentially harmful agents. Diagnosis: establishing the power of an intervention to differentiate between those with or e/o a target condition or disease Prognosis: estimating the future course of a patient’s disease.
Type of study
Treatment: Randomized control study Harm: cohort study Diagnosis: Investigators begin by collecting gp of pts suspecting of having the target condition. These pts undergoes both the new diagnostic and a gold standard Prognosis: Observational study for the prognostic factor
The hierarchy of levels of evidence
N of 1 randomized controlled trial Systematic reviews of randomized trials Single randomized trial Systematic review of observational studies addressing important pt outcomes Physiologic studies ( studies of BP, cardiac output, exercise capacity,..) Unsystematic clinical observation
The hierarchy of levels of evidence
1. A Systematic reviews/ meta-analyses B RCTs C Experimental designs 2. A Cohort control studies B Case-control studies 3.AConsensus conference B Expert opinion C Observational study D Other types of study eg. Interview based, local audit E Quasi-experimental, qualitative design 4. Personal communication
Experimental Studies or Clinical Trials
Controlled trials
are studies in which the experimental drug or procedure is compared with another drug or procedure, sometimes a placebo and sometimes the previously accepted treatment
Randomized controlled trial
provides the strongest evidence for concluding causation; it provides the best insurance that the result was due to the intervention
To reduce the chances that subjects or investigators see what they expect to see, researchers can design doubleblind trials in which neither subjects nor investigators know whether the subject is in the treatment or the control group. When only the subject is unaware, the study is called a blind trial.
Advantages of Clinical Trials
the gold standard/ causality the least number of problems or biases.
Disadvantages&
great expense long duration.
Observational Studies
case–control cross-sectional (including surveys) cohort studies case–series
case–series study
it is the simplest design in which the author describes some interesting observations that occurred for a small number of patients. not include control subjects generally not planned studies do not involve any research hypotheses important descriptive role as a precursor to other studies
Advantages & Disadvantages of Case– Series Studies
They are easy to write, and the observations may be extremely useful to investigators designing a study to evaluate causes or explanations of the observations.
Disadvantage
case–series studies are susceptible to many possible biases related to subject selection and characteristics observed hypothesis-generating and not as conclusive
Case–Control Studies
Case–control studies begin with the absence or presence of an outcome and then look backward in time to try to detect possible causes or risk factors
Advantages of Case–Control Studies
appropriate for studying rare diseases or events the quickest least expensive studies ideal for investigators who need to obtain some preliminary data prior to writing a proposal for a more complete, expensive, and time-consuming study. good choice for someone who needs to complete a clinical research project in a specific amount of time.
disadvantages
Liable to selection/ recall bias they depend completely on highquality existing records. Data availability Not for rare exposure No frequency measure
Cross-sectional studies
analyze data collected on a group of subjects at one time rather than over a period of time. designed to determine "What is happening?" right now. Subjects are selected and information is obtained in a short period of time
Advantages of Cross-Sectional Studies
Feasible Cheap Quick For screening several dis/ exposures, health planning
disadvantage
Temporal ambiguity Not for rare dis, short duration, rapidly fatal
Cohort Studies
A cohort is a group of people who have something in common and who remain part of a group over an extended time. Cohort studies ask the question "What will happen?" and thus, the direction in cohort studies is forward in time
Advantages & Disadvantages of Cohort Studies
No temporal ambiguity No selection/ recall bias Many outcomes.
advantage
Costly Timely Problem of loss to f.u
Systematic review & meta- Analysis
Summaries of evidence on a topic A review addressing a specific research question ( on treatment, Dx, prognosis, or etiology) using explicit methodology of collecting, selecting, and appraising studies and, whenever appropriate, synthesizing their results quantatively. Meta-analysis: The statistical (quantitative) combination of results from 2 or more studies addressing the same research question.
Determining Q type
Therapy: the effect of different therapy Harm: the effect of potentially harmful agents. Diagnosis: establishing the power of an intervention to differentiate between those with or e/o a target condition or disease Prognosis: estimating the future course of a patient’s disease.
Type of study
Treatment: Randomized control study Harm: cohort study Diagnosis: Investigators begin by collecting gp of pts suspecting of having the target condition. These pts undergoes both the new diagnostic and a gold standard Prognosis: Observational study for the prognostic factor
The hierarchy of levels of evidence
N of 1 randomized controlled trial Systematic reviews of randomized trials Single randomized trial Systematic review of observational studies addressing important pt outcomes Physiologic studies ( studies of BP, cardiac output, exercise capacity,..) Unsystematic clinical observation
The hierarchy of levels of evidence
1. A Systematic reviews/ meta-analyses B RCTs C Experimental designs 2. A Cohort control studies B Case-control studies 3.AConsensus conference B Expert opinion C Observational study D Other types of study eg. Interview based, local audit E Quasi-experimental, qualitative design 4. Personal communication
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