Studies of prognosis
Level	Risk of bias	Study design	Criteria
I	Low risk Study adheres to commonly held tenets of high quality design, execution and avoidance of bias	Good quality cohorta	• Prospective design • Patients at similar point in the course of their disease or treatment • F/U rate of ≥ 80%b • Patients followed long enough for outcomes to occur • Accounting for other prognostic factorsc
II	Moderately low risk Study has potential for some bias; does not meet all criteria for level I but deficiencies not likely to invalidate results or introduce significant bias	Moderate quality cohort	• Prospective design, with violation of one of the other criteria for good quality cohort study • Retrospective design, meeting all the rest of the criteria in level I
III	Moderately high risk Study has flaws in design and/or execution that increase potential for bias that may invalidate study results	Poor quality cohort Good quality case-control or cross-sectional study	• Prospective design with violation of 2 or more criteria for good quality cohort, or • Retrospective design with violation of 1 or more criteria for good quality cohort • A good case-control studyd • A good cross-sectional studye
IV	High risk Study has significant potential for bias; does not include design features geared toward minimizing bias and/or does not have a comparison group	Poor quality case-control or cross-sectional Case seriesd	• Other than a good case-control study • Other than a good cross-sectional study • Any case seriesf design

^aCohort studies follow individuals with the exposure of interest over time and monitor for occurrence of the outcome of interest.

^bApplies to cohort studies only.

^cAuthors must consider other factors that might influence patient outcomes and should control for them if appropriate.

^dA good case-control study must have the all of the following: all incident cases from the defined population over a specified time period, controls that represent the population from which the cases come, exposure that precedes an outcome of interest, and accounting for other prognostic factors.

^eA good cross-sectional study must have all of the following: a representative sample of the population of interest, an exposure that precedes an outcome of interest (e.g., sex, genetic factor), an accounting for other prognostic factors, and for surveys, at least a 80% return rate.

^fA case-series design for prognosis is one where all the patients in the study have the exposure of interest. Since all the patients have the exposure, risks of an outcome can be calculated only for those with the exposure, but cannot be compared with those who do not have the exposure. For example, a case-series evaluating the effect of smoking on spine fusion that only recruits patients who smoke can simply provide the risk of patients who smoke that result in pseudarthrosis but cannot compare this risk to those that do not smoke.