DEFINITIONS
THERAPEUTIC STUDIES investigate the results of treatment on patient outcomes and complications.
PROGNOSTIC STUDIES investigate the natural history of a disease or disorder, and they evaluate the effect of a patient characteristic on the outcome of the disease.
DIAGNOSTIC STUDIES evaluate the effectiveness of a diagnostic test or outcome instrument.
ECONOMIC/DECISION ANALYSIS OR MODELLING STUDIES explore costs and alternatives or may even develop or assess the effectiveness of decision models.
SYSTEMATIC REVIEWS AND META-ANALYSES are assigned a level of evidence equivalent to the lowest level of evidence used from the manuscripts analysed.
A PROSPECTIVE STUDY is defined as a study in which the research question was developed (and the statistical analysis for determining power was developed) before data were collected.
A RETROSPECTIVE STUDY is defined as a study in which the research question was determined after the data were collected (even for studies where the authors collected general data prospectively).
STUDY TYPE | QUESTION | LEVEL I | LEVEL II | LEVEL III | LEVEL IV | LEVEL V |
DIAGNOSTIC |
Is this (early detection) test worthwhile? |
Randomised controlled trial | Prospective cohort study | Retrospective cohort study | Case series | Mechanism-based reasoning |
Is this diagnostic or monitoring test accurate? | Testing of previously developed diagnostic criteria (consecutive patients with consistently applied reference standard and blinding) | Development of diagnostic criteria (consecutive patients with consistently applied reference standard and blinding |
Case-control study Non-consecutive patients No consistently applied reference standard |
Poor or non-independent reference standard | Mechanism-based reasoning | |
PROGNOSTIC | What is the natural history of the condition? | Inception cohort study (all patients enrolled at an early, uniform point in the course of their disease) |
Prospective cohort study (patients enrolled at different points in their disease) Control arm of randomised trial |
Retrospective cohort study
Case-control study |
Case series | Mechanism-based reasoning |
THERAPEUTIC | Does this treatment help? What are the harms? | Randomised controlled trial | Prospective cohort study
Observational study with dramatic effect |
Retrospective cohort study
Case-control study |
Case series
Historically controlled study |
Mechanism-based reasoning |
ECONOMIC | Does the intervention offer good value for money spent? | Computer simulation model (Monte Carlo simulation, Markov model) with inputs derived from Level-I studies, lifetime time duration, outcomes expressed in dollars per quality-adjusted life years (QALYs) and uncertainty examined using probabilistic sensitivity analyses | Computer simulation model (Monte Carlo simulation, Markov model) with inputs derived from Level-II studies, lifetime time duration, outcomes expressed in dollars per QALYs and uncertainty examined using probabilistic sensitivity analyses | Computer simulation model (Markov model) with inputs derived from Level-II studies, relevant time horizon, less than lifetime, outcomes expressed in dollars per QALYs and stochastic multilevel sensitivity analyses | Decision tree over the short time horizon with input data from original Level-II and III studies and uncertainty is examined by univariate sensitivity analyses | Decision tree over the short time horizon with input data informed by prior economic evaluation and uncertainty is examined by univariate sensitivity analyses |