TABLE 2.
Reporting of methods for selection of covariates1
| 2007/2008 | 2017/2018 | All articles | |
|---|---|---|---|
| (n = 53) | (n = 97) | (n = 150) | |
| Reported whether covariates were selected a priori? | |||
| Some (but not all) covariates were selected a priori | 1 (1.9) | 1 (1.0) | 2 (1.3) |
| All covariates were selected a priori | 0 (0) | 7 (7.2) | 7 (4.7) |
| Not reported | 52 (98.1) | 89 (91.8) | 141 (94.0) |
| Reported methods for selection of covariates for analysis? | |||
| Reported criteria for selection of all covariates | 9 (17.0) | 21 (21.7) | 30 (20.0) |
| Reported criteria for selection of some covariates | 10 (18.9) | 15 (15.5) | 25 (16.7) |
| Not reported | 34 (64.2) | 61 (62.9) | 95 (63.3) |
| Among studies that reported methods for selection of covariates, covariates were selected from:2 | |||
| Factors known or suspected to be associated with the exposure | 2 (3.8) | 3 (3.0) | 5 (3.3) |
| Known or established risk factors for the outcome | 13 (24.5) | 26 (26.8) | 39 (26.0) |
| Factors known or suspected to be associated with both the exposure and outcome | 1 (1.9) | 4 (4.1) | 5 (3.3) |
| Factors known or suspected to be associated with either the exposure or the outcome | 2 (3.8) | 0 (0) | 2 (1.3) |
| Confounders (factors associated with the exposure that also act on the outcome) as identified by Directed Acyclic Graphs | 0 (0) | 4 (4.1) | 4 (2.7) |
| Other | 4 (7.5) | 2 (2.1) | 6 (4.0) |
| Not reported | 34 (64.2) | 61 (62.9) | 95 (63.3) |
| Sources cited to support choice of covariates?2 | |||
| Systematic review | 1 (1.9) | 5 (5.2) | 6 (4.0) |
| Authoritative document (e.g., World Cancer Research Fund report) | 0 (0) | 4 (4.1) | 4 (2.7) |
| Narrative review | 0 (0) | 1 (1.0) | 1 (0.7) |
| Epidemiological study | 9 (17.0) | 11 (11.3) | 20 (13.3) |
| De novo literature search conducted by authors | 1 (1.9) | 9 (9.3) | 10 (6.7) |
| Methodology article | 0 (0) | 1 (1.0) | 1 (0.7) |
| No source cited | 44 (83.0) | 76 (78.4) | 120 (80.0) |
| Reported use of data-driven methods for selection of covariates for inclusion in final analytic model? | |||
| Reported use of data-driven methods for selection of all covariates for inclusion in final analytic model | 6 (11.3) | 8 (8.3) | 14 (9.3) |
| Reported use of a combination of data-driven and hypothesis-driven methods to select covariates for inclusion in final analytic model | 11 (20.8) | 15 (15.4) | 26 (17.3) |
| Did not report using any data-driven methods to select covariates | 36 (67.9) | 74 (76.3) | 110 (73.3) |
| Among studies that reported use of data-driven methods for selection of covariates, covariates were selected based on:2 | |||
| If their inclusion appreciably changed the effect estimate of the primary exposure (change-in-estimate criterion) | 11 (20.8) | 11 (11.3) | 22 (14.7) |
| P value in the final analytic model | 2 (3.8) | 1 (1.0) | 3 (2.0) |
| P value in univariate model with the exposure as the dependent variable | 1 (1.9) | 3 (3.1) | 4 (2.7) |
| P value in univariate model with the outcome as the dependent variable | 3 (5.7) | 6 (6.1) | 9 (6.0) |
| Backward elimination | 1 (1.9) | 0 (0) | 1 (0.7) |
| Stepwise selection | 0 (0) | 2 (2.1) | 2 (1.3) |
| Magnitude of correlation with exposure | 0 (0) | 1 (1.0) | 1 (0.7) |
| Whether inclusion reduced the SE of the effect estimate of the primary exposure | 1 (1.9) | 1 (1.0) | 2 (1.3) |
| Model fit3 | 0 (0) | 1 (1.0) | 1 (0.7) |
| Some description provided but unclear which specific method was used | 4 (7.5) | 3 (3.1) | 7 (4.7) |
| Did not report using any data-driven methods to select covariates | 36 (67.9) | 74 (76.3) | 110 (73.3) |
| Conducted quantitative bias analysis to evaluate impact of potential unadjusted/unmeasured confounders on results? | |||
| Yes, according to methods described by Lin et al. (29) | 0 (0) | 1 (1.0) | 1 (0.7) |
| Yes, according to methods described by Ding and VanderWeele (30) | 0 (0) | 1 (1.0) | 1 (0.7) |
| Yes, by constructing a hypothetical confounder | 1 (1.9) | 0 (0) | 1 (0.7) |
| No | 52 (98.1) | 95 (97.9) | 147 (98.0) |
1Values are n (%).
2Categories are not mutually exclusive.
3Specific measure of model fit used not reported.