Page 33 of 69 1 2 3 Figure 1. Cohort creation. 4 5 6 Periodic health examinations (PHE), 7 n = 7,591,288 8 9 10 11 Limiting to 1 index event per quarter 12 per patient, n = 7,577,428 13 14 15 16 Met exclusion criteria, n = 4,729,920 17 • Not an adult (< 18) or invalid age 18 (> 105) at time of PHE 19 20 • Invalid IKN 21 • Non-Ontario resident 22 Confidential• Long-term care resident 23 • Signs/symptoms or diagnoses of 24 cardiopulmonary disease or 25 related high-risk comorbidity 26 • Missing demographic data 27 (income quintile, sex, LHIN, or 28 29 rurality) 30 31 32 33 34 PHE cohort – multiple PHE per 35 patient, n = 2,847,508 36 37 Met multilevel regression exclusion 38 39 criteria, n = 1,138,302 40 • Could not be linked to family 41 physician and practice, 42 n = 182,948 43 • Limiting to 1 randomly selected 44 PHE per patient, n = 955,354 45 46 47 48 Patient cohort – 1 PHE per patient, 49 n = 1,709,206 50 51 52 53 54 55 56 57 58 59 60 For Peer Review Only Page 34 of 69 1 540 50000 2 3 4 530 45000 5 6 7 520 40000 8 ordered CXRs of Number 9 10 11 510 35000 12 13 14 500 30000 15 Confidential 16 17 490 25000 18 19 20 21 480 20000 22 23 24 470 15000 25 26 27 460 10000 28 29 physicians care Numberof primary 30 31 450 5000 32 33 34 440 0 35 36 0.0-1.5 1.5-2.4 2.5-5.0 >5.0 37 38 Routine CXR rate (%) 39 For Peer Review Only 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 35 of 69 1 2 3.5 180 3 4 5 160 6 3.0 1000) (x volume PHE Total 7 8 9 140 10 11 2.5 12 120 13 14 2.0 15 Confidential 100 16 17 18 80 19 1.5 20 21 60 22 use (%) CXR Routine rate 23 1.0 24 mean rate 25 40 26 phv 27 0.5 28 20 29 30 31 0.0 0 32 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 34 35 36 Quarter 37 38 39 For Peer Review Only 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 36 of 69 1 4.5% 2 3 4 5 4.0% 6 7 8 3.5% 9 10 11 12 3.0% 13 14 15 2.5% Confidential 16 17 18 19 2.0% 20 21 22 rate CXR Routine 1.5% 23 24 25 26 1.0% 27 28 29 30 0.5% 31 32 33 0.0% 34 35 1 2 3 4 5 6 7 8 9 10 11 12 13 14 36 37 LHIN 38 39 For Peer Review Only 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 37 of 69 600 1 2 3 4 500 5 6 7 8 9 10 11 400 12 13 14 15 Confidential 16 17 300 18 19 20 21 22 23 Number of practices 24 200 25 26 27 28 29 30 100 31 32 33 34 35 36 37 0 38 0.0-2.5% 2.6-5.0% 5.1-7.5% 7.6-10.0% 10.1-12.5% 12.6-15.0% 15.1-17.5% 17.6-20.0% 20.1-22.5% 22.6-25.0% >25.0% 39 For Peer Review Only 40 Routine CXR rate 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 38 of 69 1 2 STROBE Statement—checklist of items that should be included in reports of observational studies 3 4 Item Page # 5 No Recommendation 6 Title and abstract 1 (a) Indicate the study’s design with a commonly used term in the title or the 2 7 abstract 8 9 (b) Provide in the abstract an informative and balanced summary of what was 2 10 done and what was found 11 Introduction 12 Background/rationale 2 Explain the scientific background and rationale for the investigation being 3 13 14 reported 15 Objectives 3 State specific objectives, including any pre-specified hypotheses 3 16 Methods 17 Study design 4 Present key elements of study design early in the paper 4-5 18 19 Setting 5 Describe the setting, locations, and relevant dates, including periods of 4-5 20 recruitment, exposure, follow-up, and data collection 21 Participants 6 (a) CohortConfidential study—Give the eligibility criteria, and the sources and methods of 4-6 22 selection of participants. Describe methods of follow-up 23 Case-control study—Give the eligibility criteria, and the sources and methods 24 of case ascertainment and control selection. Give the rationale for the choice 25 26 of cases and controls 27 Cross-sectional study—Give the eligibility criteria, and the sources and 28 methods of selection of participants 29 (b) Cohort study—For matched studies, give matching criteria and number of N/A 30 exposed and unexposed 31 32 Case-control study—For matched studies, give matching criteria and the 33 number of controls per case 34 Variables 7 Clearly define all outcomes, exposures, predictors, potential confounders, and 5-6 35 effect modifiers. Give diagnostic criteria, if applicable 36 Data sources/ 8* For each variable of interest, give sources of data and details of methods of 4-6 37 measurement assessment (measurement). Describe comparability of assessment methods if 38 39 there is more than one group 40 Bias 9 Describe any efforts to address potential sources of bias 6-7 41 Study size 10 Explain how the study size was arrived at 4,6,8, 42 Figure 1 43 Quantitative variables 11 Explain how quantitative variables were handled in the analyses. If applicable, 6 44 45 describe which groupings were chosen and why 46 Statistical methods 12 (a) Describe all statistical methods, including those used to control for 6,7 47 confounding 48 (b) Describe any methods used to examine subgroups and interactions N/A 49 (c) Explain how missing data were addressed 5,6 50 51 Figure 1 52 (d) Describe any sensitivity analyses 9 53 54 55 56 57 58 59 1 60 For Peer Review Only Page 39 of 69 1 2 3 Results Page 4 # 5 6 Participants 13* (a) Report numbers of individuals at each stage of study—eg numbers potentially eligible, Figure 1 7 examined for eligibility, confirmed eligible, included in the study, completing follow-up, and 8 analysed 9 (b) Give reasons for non-participation at each stage Figure 1 10 (c) Consider use of a flow diagram Figure 1 11 12 Descriptive 14* (a) Give characteristics of study participants (eg demographic, clinical, social) and information 8 13 data on exposures and potential confounders 14 (b) Indicate number of participants with missing data for each variable of interest Figure 1 15 (c) Cohort study—Summarise follow-up time (eg, average and total amount) 8 16 Outcome data 15* Cohort study—Report numbers of outcome events or summary measures over time 8,9, 17 18 Table 1 19 Case-control study—Report numbers in each exposure category, or summary measures of 20 exposure 21 Cross-sectional study—Report numbers of outcome events or summary measures 22 Confidential Main results 16 (a) Give unadjusted estimates and, if applicable, confounder-adjusted estimates and their 8,9 23 24 precision (eg, 95% confidence interval). Make clear which confounders were adjusted for and Table 2 25 why they were included 26 (b) Report category boundaries when continuous variables were categorized 8,9, 27 Table 2 28 (c) If relevant, consider translating estimates of relative risk into absolute risk for a meaningful 29 30 time period 31 Other analyses 17 Report other analyses done—eg analyses of subgroups and interactions, and sensitivity analyses 9 32 Discussion 33 Key results 18 Summarise key results with reference to study objectives 10-11 34 Limitations 19 Discuss limitations of the study, taking into account sources of potential bias or imprecision. 11 35 36 Discuss both direction and magnitude of any potential bias 37 Interpretation 20 Give a cautious overall interpretation of results considering objectives, limitations, multiplicity 10-11 38 of analyses, results from similar studies, and other relevant evidence 39 Generalisability 21 Discuss the generalisability (external validity) of the study results 11 40 41 Other information 42 Funding 22 Give the source of funding and the role of the funders for the present study and, if applicable, 1 43 for the original study on which the present article is based 44 45 *Give information separately for cases and controls in case-control studies and, if applicable, for exposed and 46 unexposed groups in cohort and cross-sectional studies. 47 48 Note: An Explanation and Elaboration article discusses each checklist item and gives methodological background and published 49 examples of transparent reporting. The STROBE checklist is best used in conjunction with this article (freely available on the Web 50 sites of PLoS Medicine at http://www.plosmedicine.org/, Annals of Internal Medicine at http://www.annals.org/, and Epidemiology 51 at http://www.epidem.com/). Information on the STROBE Initiative is available at www.strobe-statement.org.