Ostergaard 1998.

Methods	Setting: High schools in Aarhus, Denmark Study design: Cluster randomized trial, parallel two arms. Enrollment between January 1997 and April 1997 Sample size estimation a priori: No
Participants	Inclusion criteria Male and female students who returned the questionnaire and were sexually experienced Exclusion criteria Not specified Population Mean age females 18 years, males 18.2 years. 8909 students were randomized, 4336 to home‐based strategy (2603 women and 1733 men) and 4573 to clinic‐based strategy (2884 women and 1689 men) for CT detection. To deal with unit analysis issues about cluster‐randomized studies we used an ICC 0,00119, average cluster size of 524; hence the effective sample size for home group was 2673 and 2819 for clinic group.
Interventions	Home‐based specimen collection (n = 2673) In the home group the students completed a questionnaire and received information regarding CT infection; female students were asked to collect two urine samples and one vaginal flush sample, males were asked to collect one first void urine sample and mail from home to the laboratory. Clinical‐based specimen collection (n = 2819) In the clinic group the students completed a questionnaire and received information regarding CT infection; they were offered a free test from their doctor or at the local clinic.
Outcomes	Number of students tested (799/2673)(41/2819) Number of students infected (33/799)(4/41)
Notes	Funding sources: "The study was funded by the Danish National Board of Health, Løvens Kemiske Fabriks Research Foundation, Nycomed DAK, Pfizer, and Chairman Jacob Madsen and Hustru Olga Madsen's foundation" Role of funder: Not reported Declarations of interest: None declared It is unclear if all participants with positive test and their sexual partners were treated. Ethical approval was obtained from ethics committee and the Danish Data Protection Agency. Unclear whether consent was obtained Number of identifier register: Not found
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote "We randomised all 17 high schools": details about the process of sequence generation for cluster‐randomization not reported
Allocation concealment (selection bias)	Unclear risk	No details on allocation concealment given in the report.
Blinding of participants and personnel (performance bias) All outcomes	High risk	No blinding, the outcomes are likely to be influenced by lack of blinding.
Blinding of outcome assessment (detection bias) Subjective outcomes	Unclear risk	No subjective outcomes reported.
Blinding of outcome assessment (detection bias) Objective outcomes	Unclear risk	It is not reported if the laboratory personnel or assessors were blinded.
Incomplete outcome data (attrition bias) Short‐term outcomes	High risk	After we used the effective sample size for this cluster‐randomized study, the attrition rate was 799/2673 for home‐based group; and for clinic‐based group it was 41/2819. More than 20% participants were not tested and the missing data were not balanced in the groups.
Incomplete outcome data (attrition bias) Long‐term outcomes	Unclear risk	Only number of individuals not tested reported; however clearly information was not available to make a judgement.
Selective reporting (reporting bias)	Unclear risk	Protocol was not available from 3 trial registries. No information available in methods section to make a judgement.
Other bias	Unclear risk	No other bias identified.