Dickersin 1992.

Study characteristics
Methods	Studies submitted and approved by two institutional review boards (IRBs) which serve the John Hopkins Health Institutions prior to and during 1980. Publication status was obtained in 1988 by a telephone call to the principal investigator of each study.
Data	168 clinical trials.
Comparisons	Publication status of studies with significant findings compared with those with non‐significant findings. Studies were classified as either statistically significant if the P value was < 0.05, or as not significant. When statistical tests were not used, investigators were asked to classify the findings as "important" or not.
Outcomes	Trials with significant findings were more likely to be published than those showing non‐significant findings. Total published = 136/168 (81%) Significant = 84/96 (87%) Non significant = 52/72 (72%) Other variables assessed included sample size, primary funding source, sex and academic rank.
Notes	1048 applications were received by the IRBs of which 514 were included in the analysis. Reasons for exclusion were: applications withdrawn, not approved, not implemented, exempt, did not describe a study, or no humans (n = 311); data on both results and publication not available (n = 223). 273 were observational studies, 73 were experimental studies and 168 were clinical trials and included data on both study results and publication.
Risk of bias
Item	Authors' judgement	Support for judgement
Inception cohort?	Yes	"The studies that formed the basis for our research were those that appeared on the logs of the two institutional review boards (IRBS) that serve The Johns Hopkins Health Institutions and were approved in 1980 or prior to 1980 and were still ongoing in that year... the logs of the two institutions ... enumerated 1048 applications." (page 374)
Complete follow up of all trials?	Yes	1048 applications were received by the IRBs of which 514 were included in the analysis. Reasons for exclusion were: applications withdrawn, not approved, not implemented, exempt, did not describe a study, or no humans (n = 311); data on both results and publication not available (n = 223). 273 were observational studies, 73 were experimental studies and 168 were clinical trials.
Publication ascertained through personal contact with investigators or sponsor?	Yes	"The principal investigators associated with interview eligible studies were contacted for interviews in 1988 ... Publication status of a study was determined from responses provided to specific questions asked during the interview." (page 375)
Definition of positive and negative findings clearly defined?	Yes	"Studies reported to have statistically significant findings were combined with those reported to have findings of great importance. Together they are referred to as "significant" and are contrasted with the reminder, which are referred to as "not significant”. In this article we chose to use the term statistically significant to refer to P value less than 0.05." (page 375)
Possible confounders controlled for in the analysis	Yes	" ... initially, unadjusted ORs for the association between variables listed in table 4 and publication were calculated for each IRB separately using SAS ... Subsequently, adjusted ORs for each IRB alone and for the two IRBs combined (by including a term in the model for the effect of IRB) were calculated using multiple logistic regression. The combined model included two‐way interaction terms between IRB and each of the other factors." (page 376).