In a case-control study,
A. Subjects are selected on the basis of having a disease (cases, controls)
B. Groups are compared with respect to the proportion of having a history of exposure
C. Advantages: relatively quick and inexpensive; suitable for the evaluation of the diseases with long latent periods and for rare diseases; can assess multiple risk factors for a single disease
Disadvantages: cannot directly calculate the incidence of disease unless the study is population-based; difficult to establish temporal sequence and causal association between exposure and outcome;prone to recall bias; inefficient for rare exposures
A cohort study:
A. Can be prospective (disease has not occurred at the beginning of the study) and retrospective (disease has occurred at the time the study is initiated)
B. Subjects are selected on the basis of having the exposure and are followed up over time to assess the outcome; it is direct measurement of disease incidence
C. Advantages: can directly calculate the incidence of disease among exposed and non-exposed; can elucidate temporal relationship between exposure and outcome; bias in exposure assessment is minimized (prospective studies)
Disadvantages: expensive and time-consuming (especially if prospective); inefficient for rare diseases; requires availability of adequate records (retrospective); losses to follow-up may substantially affect the validity of the results
Disadvantages of randomized clinical trials are:
A. ethical concerns
B. costs
C. feasibility
D. non-compliance
One of the commonly used systems for grading the evidence is:
A. Ia: systematic review or meta-analysis of RCTs; Ib: at least one RCT
B. IIa: at least one well-designed controlled study without randomization; IIb: at least one well-designed quasi-experimental study, such as a cohort study
C. III: well-designed non-experimental descriptive studies (comparative studies, correlation studies, case-control studies, case series, case reports)
D. IV: expert committee reports, opinions formal consensus
The levels of recommendation are based on the strength of evidence:
A: based on hierarchy I evidence
B: based on hierarchy II evidence or extrapolated from hierarchy I evidence
C: based on hierarchy II evidence or extrapolated from hierarchy I or II evidence
D: directly based on hierarchy IV evidence or extrapolated from hierarchy I, II, or III evidence
微信支付 
支付宝支付