Cohort Studies

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Cohort Studies : 

Cohort Studies Dr. P. V. Kotecha Professor & Head Preventive and Social Medicine Government Medical College Vadodara Gujarat, India pvkotecha@gmail.com

Basic Concept : 

2/19/2009 Dr. P. V. Kotecha Basic Concept The basic aim of epidemiological research is to investigate the association between exposure to a suspected risk factor and the occurrence of disease. This requires that the incidence of disease in a group of persons exposed to thesuspected risk factor is compared with the incidence in a group of persons not exposed.

Basic Concept : 

2/19/2009 Dr. P. V. Kotecha Basic Concept This comparison can be summarized by calculating either the ratio of the measures of disease frequency for the exposed and unexposed two groups, which indicates the likelihood of developing the disease in the exposed individuals relative to those unexposed, or the difference between the two, which provides information about the absolute effect of the exposure in those exposed compared with those unexposed.

Definition : 

2/19/2009 Dr. P. V. Kotecha Definition A cohort study is an observational study in which a group of persons exposed and a group of persons unexposed to a potential cause and disease are followed up in time and the incidence of the disease in one group is compared with the incidence in the other. Thus cohort study is not a study of A cohort but requires two or more cohorts…?

Direction of study : 

2/19/2009 Dr. P. V. Kotecha ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Direction of study ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

This is where the study begins : 

2/19/2009 Dr. P. V. Kotecha This is where the study begins

This is how the study may end : 

2/19/2009 Dr. P. V. Kotecha This is how the study may end

This is how the real situation might be….. : 

2/19/2009 Dr. P. V. Kotecha This is how the real situation might be…..

Measurement that it gives : 

2/19/2009 Dr. P. V. Kotecha Measurement that it gives Incidence risk in the exposed Incidence risk in the non exposed Relative Risk among exposed as compared to not exposed Risk Difference (Attributable Risk) Etiological Fraction (Attributable risk percentage) If you know the percentage exposed you can get population attributable risk

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Incidence risk in the exposed 250/1000= 25% or 0.25 as risk usually is displayed Incidence risk in the non exposed 100/1000=10% or0.1 as risk usually is displayed

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Relative Risk is a relative figure of comparison. This is the ratio of risk among exposed to risk among not exposed = 0.25/0.1=2.5 This means that the risk is 2.5 times more among those who are exposed as compared to those not exposed

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Risk Difference (Attributable Risk) is the risk difference among exposed and non exposed cohort This means 0.25-0.1= 0.15 out of 0.25 risk is attributed to exposure status

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Etiological Fraction (Attributable risk percentage) This mentions how much of the fraction of total risk experienced among those exposed is attributed to being exposed.. In this case 0.25-0.10 is due to exposure out of total 0.25 i.e. 0.25-0.1 =0.6 or 60% 0.15 are getting due to their being exposed

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Relative Risk is a measurement useful for researchers. It gives relative importance of the risk factor being studied. Thus it also gives etiological importance of it. Higher the relative risk, more important is that risk factor in causing the disease. Reverse need not be true….. ?

Measurement : 

2/19/2009 Dr. P. V. Kotecha Measurement Attributable risk is directly of importance to public health people. More the attributable risk, more its importance for prevention from public health perspective. At some stage it also depends upon the prevalence of risk factors in the community…

Advantages of Cohort Study : 

2/19/2009 Dr. P. V. Kotecha Advantages of Cohort Study Exposure is measured before the onset of the disease and is therefore unbiased in terms of disease development Rare exposure can be examined by appropriate selection of the study cohort Multiple outcomes of the diseases can be studied. Incidence of the disease can be measured in exposed and unexposed

Essential Elements: : 

2/19/2009 Dr. P. V. Kotecha Essential Elements: Formulation of clear hypothesis Identification of the groups of individuals to be studied Definition of exposure and its measurement Measurements of confounders Definition of outcome variable Analysis Interpretation

Disadvantage of Cohort Study : 

2/19/2009 Dr. P. V. Kotecha Disadvantage of Cohort Study Expensive and time consuming Changes over time in exposure status and in diagnostic criteria can affect the classification of the individuals Ascertainment of outcome may be influenced by exposure Losses to follow up may introduce bias

Intricacies…. : 

2/19/2009 Dr. P. V. Kotecha Intricacies….

Intricacies…. : 

2/19/2009 Dr. P. V. Kotecha Intricacies….

Questions? : 

2/19/2009 Dr. P. V. Kotecha Questions?

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