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Table 2 Methodological Characteristics of Mediation Analyses Performed Based on Observational Epidemiologic Studies Published Between 2015 and 2019

From: Mediation analysis methods used in observational research: a scoping review and recommendations

Methodological characteristics Overall (n = 174) Causal steps, change-in-coefficient, and joint significance (n = 28) Traditional mediation analysis (n = 123) Causal mediation analysis
(n = 23)
Year published, n (%)a
 2015 21 (12.1) 4 (14.3) 16 (13.0) 1 (4.3)
 2016 29 (16.7) 8 (28.6) 16 (13.0) 5 (21.7)
 2017 27 (15.5) 2 (7.1) 21 (17.1) 4 (17.4)
 2018 47 (27.0) 6 (21.4) 34 (27.6) 7 (30.4)
 2019 50 (28.7) 8 (28.6) 36 (29.3) 6 (26.1)
Study design, n (%)a
 Cross-sectional 84 (48.3) 15 (53.6) 64 (52.0) 5 (21.7)
 Case-control 7 (4.0) 1 (3.6) 3 (2.4) 3 (13.0)
 Prospective cohort 78 (44.8) 12 (42.9) 53 (43.1) 13 (56.5)
 Retrospective cohort 5 (2.9) 0 (0.0) 3 (2.4) 2 (8.7)
 Analytical sample size, median (IQR) 428.5 (157.5–2026.0) 443.5 (146.5–1442.3) 326.0 (147.0–935.0) 2249.0 (703.0–9484.0)
Software program used, n (%)a
 SAS 21 (12.1) 4 (14.3) 9 (7.3) 8 (34.8)
 Stata 27 (15.5) 5 (17.9) 18 (14.6) 4 (17.4)
 SPSS 67 (38.5) 13 (46.4) 54 (43.9) 0 (0.0)
 Mplus 26 (14.9) 1 (3.6) 25 (20.3) 0 (0.0)
 R 14 (8.0) 0 (0.0) 5 (4.1) 9 (39.1)
 LISREL 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
 Unclear, multiple programs used 5 (2.9) 1 (3.6) 4 (3.3) 0 (0.0)
 Not mentioned 13 (7.5) 4 (14.3) 7 (5.7) 2 (8.7)
Number of exposures, n (%)a
 1 116 (66.7) 19 (67.9) 81 (65.9) 16 (69.6)
 2 35 (20.1) 3 (10.7) 26 (21.1) 6 (26.1)
 3 or more (max. 14) 23 (13.2) 6 (21.4) 16 (13.0) 1 (4.3)
Type of exposure, n (% of studies)b
 Continuous 105 (60.3) 15 (53.6) 84 (68.3) 6 (26.1)
 Binary 51 (29.3) 8 (28.6) 28 (22.8) 15 (65.2)
 Categorical 20 (11.5) 5 (17.9) 11 (8.9) 4 (17.4)
 Latent 10 (5.7) 2 (7.1) 8 (34.8) 0 (0.0)
Number of mediators, n (%)a
 1 86 (49.4) 14 (50.0) 59 (48.0) 13 (56.5)
 2 35 (20.1) 5 (17.9) 28 (22.8) 2 (8.7)
 3 or more (max. 19) 53 (30.5) 9 (32.1) 36 (29.3) 8 (34.8)
Type of mediator, n (% of studies)b
 Continuous 136 (78.2) 19 (67.9) 103 (83.7) 14 (60.9)
 Binary 31 (17.8) 10 (35.7) 13 (10.6) 8 (34.8)
 Categorical 9 (5.2) 3 (10.7) 3 (2.4) 3 (13.0)
 Count 1 (0.6) 0 (0.0) 0 (0.0) 1 (4.3)
 Time-to-event 2 (1.1) 0 (0.0) 2 (1.6) 0 (0.0)
 Latent 10 (5.7) 3 (10.7) 7 (5.7) 0 (0.0)
Number of outcomes, n (%)a
 1 126 (72.4) 22 (78.6) 88 (71.5) 16 (69.6)
 2 26 (15.0) 2 (7.1) 22 (17.9) 2 (8.7)
 3 or more (max. 8) 22 (12.6) 4 (14.3) 13 (10.6) 5 (21.6)
Type of outcome, n (% of studies)b
 Continuous 110 (63.2) 18 (64.3) 86 (69.9) 6 (26.1)
 Binary 47 (27.0) 6 (21.4) 27 (22.0) 14 (60.9)
 Categorical 6 (3.4) 2 (7.1) 3 (2.4) 1 (4.3)
 Count 4 (2.3) 0 (0.0) 2 (1.6) 2 (8.7)
 Time-to-event 8 (4.6) 3 (10.7) 3 (2.4) 2 (8.7)
 Latent 10 (5.7) 3 (10.7) 7 (5.7) 0 (0.0)
Diagram of model included, n (%)a
 Yes 130 (74.7) 14 (50.0) 106 (86.2) 10 (43.5)
 No 44 (25.3) 14 (50.0) 17 (13.8) 13 (56.5)
Repeated measurements, n (%)a
 Yes 41 (23.6) 6 (21.4) 28 (22.8) 7 (30.4)
 No 132 (75.9) 22 (78.6) 94 (76.4) 16 (69.6)
 Unclear 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
Type of mediation model, n (%)a
 Single mediator 114 (65.5) 20 (71.4) 74 (60.2) 20 (87.0)
 Multiple mediator 41 (23.6) 5 (17.9) 34 (27.6) 2 (8.7)
 Both single and multiple mediator 16 (9.2) 3 (10.7) 12 (9.8) 1 (4.3)
 Unclear 3 (1.7) 0 (0.0) 3 (2.4) 0 (0.0)
Regression for the mediator equation, n (% of studies)b
 Linear 122 (70.1) 14 (50.0) 98 (79.7) 10 (43.4)
 Logistic 15 (8.6) 2 (7.1) 7 (5.7) 6 (26.1)
 Poisson 1 (0.6) 0 (0.0) 0 (0.0) 1 (4.3)
 Cox proportional hazards 2 (1.1) 0 (0.0) 2 (1.6) 0 (0.0)
 Multilevel linear 7 (4.0) 0 (0.0) 7 (5.7) 0 (0.0)
 Multilevel logistic 1 (0.6) 1 (3.6) 0 (0.0) 0 (0.0)
 Unclear 16 (9.2) 2 (7.1) 8 (6.5) 6 (26.1)
 Not estimated 12 (6.9) 9 (32.1) 3 (2.4) 0 (0.0)
Regression for the outcome equation, n (% of studies)b
 Linear 109 (62.6) 17 (60.7) 87 (70.7) 5 (21.7)
 Logistic 39 (22.4) 5 (17.9) 23 (18.7) 11 (47.8)
 Probit 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
 Log-linear 1 (0.6) 0 (0.0) 0 (0.0) 1 (4.3)
 Multinomial logistic 1 (0.6) 1 (3.6) 2 (1.6) 0 (0.0)
 Poisson 3 (1.7) 0 (0.0) 0 (0.0) 3 (13.0)
 Negative binomial 3 (1.7) 0 (0.0) 1 (0.8) 2 (8.7)
 Cox proportional hazards 10 (5.7) 4 (14.3) 4 (3.3) 1 (4.3)
 Additive hazards model 1 (0.6) 0 (0.0) 0 (0.0) 1 (4.3)
 Multilevel linear 9 (5.2) 2 (7.1) 7 (5.7) 0 (0.0)
 Multilevel logistic 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
 Multilevel log-linear 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
 Unclear 3 (1.7) 1 (3.6) 1 (0.8) 1 (4.3)
Type of confidence interval for the indirect effect, n (%)a
 Normal-based 7 (4.0) 0 (0.0) 6 (4.9) 1 (4.3)
 Percentile bootstrap 7 (4.0) 0 (0.0) 5 (4.1) 2 (8.7)
 Bias-corrected bootstrap 35 (20.1) 0 (0.0) 35 (28.5) 0 (0.0)
 Bias-corrected and accelerated bootstrap 2 (1.1) 0 (0.0) 2 (1.6) 0 (0.0)
 Percentile and bias-corrected and accelerated bootstrap 1 (0.6) 0 (0.0) 1 (0.8) 0 (0.0)
 Bootstrap, not specified 36 (20.7) 1 (3.6) 28 (22.8) 7 (30.4)
 Distribution of the product 3 (1.7) 0 (0.0) 3 (2.4) 0 (0.0)
 Monte Carlo 2 (1.1) 0 (0.0) 2 (1.6) 0 (0.0)
 Bayesian credible intervals 3 (1.7) 0 (0.0) 3 (2.4) 0 (0.0)
 Unclear 18 (10.3) 0 (0.0) 8 (6.5) 10 (43.5)
 Not reported 60 (34.5) 27 (96.4) 30 (24.4) 3 (13.0)
Standard error for indirect effect, n (%)a
 Yes 37 (21.3) 0 (0.0) 35 (28.5) 2 (8.7)
 No 137 (78.7) 28 (100.0) 88 (71.5) 21 (91.3)
Statistical test for indirect effect, n (%)a
 Yes 62 (35.6) 7 (25.0) 46 (37.4) 9 (39.1)
 No 112 (64.4) 21 (75.0) 77 (62.6) 14 (58.3)
Effect size measure, n (%)a
 Proportion mediated 65 (37.4) 3 (10.7) 43 (35.0) 19 (82.6)
 Standardized effect 6 (3.4) 0 (0.0) 6 (4.9) 0 (0.0)
 No effect size measure 103 (59.2) 25 (89.3) 74 (60.2) 4 (17.4)
Inclusion of confounders, n (%)a
 Yes 125 (71.8) 19 (67.9) 88 (71.5) 18 (78.3)
 No 41 (23.6) 8 (28.6) 31 (25.2) 2 (8.7)
 Unclear 8 (4.6) 1 (3.6) 4 (3.3) 3 (13.0)
Sensitivity analyses for unmeasured confounders, n (%)a
 Yes 3 (1.7) 0 (0.0) 0 (0.0) 3 (13.0)
 No 170 (97.7) 28 (100.0) 123 (0.0) 19 (82.6)
 No, but discussed as unnecessary 1 (0.6) 0 (0.0) 0 (0.0) 1 (4.3)
Effect modifiers considered, n (%)a
 Yes, a priori stratification 10 (5.7) 3 (10.7) 5 (4.1) 2 (8.7)
 Yes, through an interaction term 28 (16.1) 6 (21.4) 19 (15.4) 3 (13.0)
 No 136 (78.2) 19 (67.9) 99 (80.5) 18 (78.3)
Assessment of exposure-mediator interaction, n (%)a
 Yes 17 (9.8) 3 (10.7) 5 (4.1) 9 (39.1)
 No 157 (90.2) 25 (89.3) 118 (95.9) 14 (60.9)
  1. a The percentages are computed based on the column frequencies
  2. b Because some studies considered multiple exposure, mediator, and outcome variables, the total number is higher than the number of studies. The percentages reflect the percentages of the total number of studies, e.g., 60.3% of the studies included analyses with a continuous exposure variable versus 39.7% which did not include analyses with a continuous exposure variable