The size of the bias depends on
The size of the bias depends on the proportion of patients excluded by non-consent and by how this selection influences this group, which will vary with each registry. Thus, in an observational study of breast cancer in the UK, it was demonstrated that the use of the dataset requiring consent omitted information about many women with locally advanced or metastatic cancer, and also underrepresented women in deprived social groups . A clinical registry of acute renal disease in the US requiring no interventions but requiring informed consent only achieved 52% completeness . A major multicentre effort to record information of the natural history and treatment of stroke in Canada failed because of the requirement for informed consent, so that initially only 39% of eligible patients, and after simplification of the requirements only 50% of eligible patients, give informed consent for the data to be used; and the costs involved in obtaining consent formed a high proportion of the total costs of the endeavour. The authors argued that the avoidance of informed consent from minimal risk observational research is essential . For a planned population-based cohort study of patients with three common cancers requiring further information on therapy and care from eligible patients, only 47% of patients give consent; so while internal comparisons within the group may be valid, the whole group is not, as planned, a representative population-based study . A small study requiring consent from the review of medical records of men with prostate cancer achieved in 84% response rate, but at a considerable cost . Some FF-MAS groups are particularly difficult to reach. To study unmet needs, in Australia an attempt was made to contact a representative sample of adolescent and young adult survivors of cancer. Initial identification could easily be made from standard existing registries, but an active clinical consent protocol was used by the registries approaching subjects through their clinicians. As a result the overall consent rate was only 7.8%, due mainly to non-responses from clinicians or contact not being established. In contrast very few individuals who were approached refused personal consent . Data completeness has also been threatened on a larger scale. In regions of Germany, law changes in the 1980s requiring informed consent for cancer registries which previously had included all cases of cancer without consent for over 50 years. This quickly resulted in at least 30% failure of registration, and eventual failure of the registries as they were no longer valuable by being incomplete. Subsequent attempts to improve this led to a complicated system using two centres produced duplicate registrations and continued failure . Changes in regulations by large jurisdictions such as the European Union can threaten the completeness or existence of registration and essential linkages . Variations in administrative decisions and ethical committee decisions can threaten registration issues . Even in the same legislative climate, with a common protocol, different organisations can vary greatly on how patients are approached for consent and registration . The efficiency and transparency in the operation of administrative structures and ethical committees also needs considered, particularly if several jurisdictions are involved . It has been noted that “those responsible for framing guidelines on the handling of clinical data and for advising doctors should consider issues related to health surveillance so as public health is not put at risk” . A restrictive policy is not supported by the general public. In a survey of 2872 respondents in the UK in 2005, 95% considered the existence of a national cancer registry with enough detail to assess effectiveness of treatment as useful, and 81% supported legal registration of all cases of cancer to such a registry . Only 16% had any objection to being identified and approached for further study from such a registry.