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Table 1 Study Characteristics That Influence Results

From: Assessing observational studies of medical treatments

Characteristics That Influence Applicability
   Treatment specifics
Details of procedure
Details of ancillary treatments or management of intermediate outcomes
Method of patient contact and assessment at follow-up
Length of follow-up
   Patient characteristics
Study setting
Inclusion/exclusion criteria
Reported characteristics
Characteristics That Influence Validity
   Information bias
Incorrectly ascertaining treatment or outcome
   Selection bias
Pretreatment – subjects on different treatments have different risks
Post treatment – lost to follow-up depends on outcome and treatment
Caused by pretreatment selection bias
Demonstrated by differences in risk factors between treatment groups
Possibly reduced by risk-adjustment