Burditt 1968.

Study characteristics
Methods	Study design: retrospective case‐control longitudinal Location: UK Time period: late 1949 to 1968 Outcome: PDR Outcome measurement: incidence
Participants	Number of participants: 2184 Diabetes type: unclear Inclusion criteria: diagnosis of DM (random blood sugar > 180 mg/100 ml2; fasting blood sugar > 130 mg/100 ml2; glucose tolerance test ‘florid diabetes’; other evidence e.g. use of insulin) Exclusion criteria: DM known to be associated with chronic pancreatitis, haemochromatosis, or acromegaly
Notes	Link to data extraction table, risk of bias assessment, and results: osf.io/2b9q3/?view_only=eb515e7e72f3433b80fc94df4714b53f
Item	Authors' judgement	Support for judgement
Study participation	Unclear	See ‘Notes’ above for link to risk of bias assessment
Study attrition	No	See ‘Notes’ above for link to risk of bias assessment
Prognostic factor measurement	Unclear	See ‘Notes’ above for link to risk of bias assessment
Outcome measurement	Unclear	See ‘Notes’ above for link to risk of bias assessment
Adjustment for other prognostic factors	No	See ‘Notes’ above for link to risk of bias assessment
Statistical analysis and reporting	Unclear	See ‘Notes’ above for link to risk of bias assessment