Question

answer the following questions

Stage 1 : categorized the evidence by the Quality of its source.

1 . Trails ( planned interventions with contemporaneous assignment of treatment and nontreatment

( a) Randomized , double -blinded, placebo -controlled with sufficient power appropriately analyzed

(b) Randomized but blindness not achieved.

(c) Nonrandomized trails with good control of confounding , that are well conducted in other respects

(d)Randomized but with deficiencies in execution or analysis ( insufficient power , major losses to follow-up , suspect randomization , analysis with exclusions)

(e) Nonrandomized trails with deficiencies in execution or analysis

2. Cohort or case- control studies

(a) Hypothesis specified before analysis , good data , confounders accounted for

(b) as above but hypothesis not specified before analysis

(c) post hoc, but hypothesis not specified before analysis

3. Time -series studies

(a) analyses that take confounding into account

(b) analyses that do not consider confounding

4. Case-series studies : series of case reports without specific comparison group

(A) among other issues that must be considered in reviewing the evidence are the precision of definition of the outcome being measured , the degree to which the study methodology has been described , adequacy of the sample size , and the degree to which characteristics of the population studied and of the intervention being evaluated have been described.

(b) A study can be well designed and carried out in exemplary fashion (internal validity) , but if the population studied is an unusual or highly selected one , the results may not be generalizable (external validity)

STAGEII: guidelines for evaluating the evidence of a causal relationship.

1. Major criteria

(a) temporal relationship: an intervention can be considered evidence of a reduction in risk of disease or abnormality only if the intervention was applied before the time disease or abnormality would have developed.

(b) biologic plausibility : a biological plausible mechanism should be able to explain why such a relationship would be be expected to occur

(c)consistency : single studies are rarely definitive. Study findings are replicated in different populations and by different investigators carry more weigh than those that are not . If the findings of studies are inconsistent , the inconsistency must be explained.

(d)alternative explanations ( confounding):The extent to which alternative explanations have been explored is an important criterion in judging causality.

2. Other considerations

(a) dose - response relationship: if a factor is indeed the cause of a disease , usually ( but not invariably ) the greater the exposure to the factor , the greater the risk of the disease. Such a dose - response relationship may not always be seen because many important biologic relationships are dichotomous and reach a threshold level for observed effects

(b)strength of association: The strength of the association is usually measured by the extent to which the relative risk or odds depart from unity , either greater 1 ( in the case of disease- causing exposures) or less than 1 ( in the case of preventive interventions).

(c) cessation effects : if an intervention has a beneficial effect, the effect should cease when it is removed from a population ( unless carryover effect is operant)

CLASS- EPIDEMOLOGY