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	Types of Studies
	Therapeutic Studies – Investigating the results of treatment	Prognostic Studies – Investigating the effect of a patient characteristic on the outcome of disease	Diagnostic Studies – Investigating a diagnostic test	Economic and Decision Analyses – Developing an economic or decision model
Level I	High quality randomized trial with statistically significant difference or no statistically significant difference but narrow confidence intervals Systematic Review ² of Level I RCTs (and study results were homogenous ³)	High quality prospective study ⁴ (all patients were enrolled at the same point in their disease with ≥ 80% follow up of enrolled patients) Systematic review ² of Level I studies	Testing of previously developed diagnostic criteria on consecutive patients (with universally applied reference “gold” standard) Systematic review ² of Level I studies	Sensible costs and alternatives; values obtained from many studies; with multiway sensitivity analyses Systematic review ² of Level I studies
Level II	Lesser quality RCT (e.g. < 80% follow up, no blinding, or improper randomization) Prospective ⁴ comparative study ⁵ Systematic review ² of Level II studies or Level 1 studies with inconsistent results	Retrospective ⁶ study Untreated controls from an RCT Lesser quality prospective study (e.g. patients enrolled at different points in their disease or <80% follow-up.) Systematic review ² of Level II studies	Development of diagnostic criteria on consecutive patients (with universally applied reference “gold” standard) Systematic review ² of Level II studies	Sensible costs and alternatives; values obtained from limited studies; with multiway sensitivity analyses Systematic review ² of Level II studies
Level III	Case control study7 Retrospective ⁶ comparative study ⁵ Systematic review ² of Level III studies	Case control study ⁷	Study of nonS consecutive patients; without consistently applied reference “gold” standard Systematic review ² of Level III studies	Analyses based on limited alternatives and costs; and poor estimates Systematic review ² of Level III studies
Level IV	Case Series ⁸	Case series	Case-control study Poor reference standard	Analyses with no sensitivity analyses
Level V	Expert Opinion	Expert Opinion	Expert Opinion	Expert Opinion

A complete assessment of quality of individual studies requires critical appraisal of all aspects of the study design.
A combination of results from two or more prior studies.
Studies provided consistent results.
Study was started before the first patient enrolled.
Patients treated one way (e.g. cemented hip arthroplasty) compared with a group of patients treated in another way (e.g. uncemented hip arthroplasty) at the same institution.
The study was started after the first patient enrolled.
Patients identified for the study based on their outcome, called “cases”; e.g. failed total arthroplasty, are compared to those who did not have outcome, called “controls”; e.g. successful total hip arthroplasty.
Patients treated one way with no comparison group of patients treated in another way.

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Levels of evidence as adopted by ASSI

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