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This paper deals with the main security services of data protection: integrity, authentication, nonrepudiation and confidentiality. It describes the different tools which may be used to achieve these security objectives: encryption, digital signature, access controls, integrity control, audit and certification. The French legislation concerning encryption has been recently updated in order to simplify the use of encryption techniques for personal data security. This legislation describes different authorization or reporting procedures according to the type of the request (application, equipment, import or export of encryption techniques) and the security level required.

Record linkage, for compiling sameperson records from various source files, can improve the feasibility of epidemiological research using populationbased studies. The question is comply with the European legislation on data privacy and data security. For example, a computerized record hash coding and linkage procedure is described to link medical information within the framework of epidemiological followup. Before their extraction, files are rendered anonymous using a oneway hash coding based on the standard hash algorithm (SHA) function. Once rendered anonymous using the software ANONYMAT, the linkage of patient information can be accomplished by means of a mixture model, taking into account several identification variables. An application of this anonymous record linkage procedure was carried out in order to link medical files on cancer, from 3 hospitals of the French RhôneAlpes region. This application stresses how the use of the ANONYMAT software allows compliance with the legislation on data confidentiality without entailing problems on data availability.

After the implementation of the Medicare Prospective Payment System (PPS) in the USA, many European countries like France have introduced DRGs to curb hospital overspending. However, there has been some reluctance from hospital actors, especially because of the heterogeneous nature of DRG's. To analyse this situation, we propose a method based on distribution modelization of length of stays and costs within DRGs. For each DRG, the model is based on a mixture of Poisson and Weibull distributions identified as subgroups. The subgroups are characterized by their means and their proportions which are estimated by maximization of data likelihood. For a particular DRG, the proportion of long stay or high-cost patients can be explained by the introduction of clinical variables in the model. First the model was applied to the DRG "leukemia and lymphoma" (HCFA V.3), using 133 discharge abstract files from the Dijon public teaching hospital which were classified into this DRG in 1993. Among the studies parameters only acute leukemia, neutropenia < 500 PNN/mm3, high dose aplastic chemotherapy, central venous catheterization, parenteral nutrition, use of protected or laminar air flow room, septicemia, large spectrum intravenous antibiotherapy, and blood transfusion had a significant influence on the distribution of the patients in the long stay or costly subgroup. Second, for DRG "chronic bronchopneumopathies" (n = 220) the significant parameters were mechanical ventilation, antibiotherapy, post hospitalization medicalized care.

Since 1985, cases of acute vascular cerebral diseases observed among the inhabitants of the city of Dijon have been systematically recorded: 88 cerebral haemorrhages were reported from 1985 to 1988. Twenty-five variables have been defined from antecedents, clinical and para-clinical data. Seven variables, were statistically associated with death at the end of the first month: age, severity of neurological palsy, meningeal syndrome, coma or comatose state, hyperglycemia and mass effect on CAT-scan. Using a step-down variable selection procedure, two prognostic factors were found: loss of consciousness and advanced age.

Beginning in 1985, the Dijon Stroke Registry has identified every case of stroke occurring in this city of 140,000 inhabitants. The systematic use of a CT scan and multiple other sources of information ranging from hospital doctors to general practitioners make it possible for us to have virtually exhaustive data on stroke incidence. The gross incidence was 170 per 100,000 among men and 126 per 100,000 among women. Specific incidence by age and sex shows female predominance up to 30 years of age, and male predominance among victims from 30 to 80 years old. Beyond 80, rates were identical for both sexes. In 68% of the cases, the strokes were caused by infarct; in 12%, by lacunae; in 15%, by cerebral hemorrhage; in 5%, by subarachnoid hemorrhage. Significant differences existed in this distribution according to age, with a clear predominance of young people among those having subarachnoid hemorrhage, while, for those over 40, cerebral hemorrhages were frequent; the proportion of infarct increased with age for both sexes. A peak of incidence for infarct appeared during winter, while transient ischemic accident was more frequent in summer. Cerebral hemorrhage had a similar incidence throughout the year. Mortality was high during the first month (12.5% during the first week, 21.5% during the first month), and reached 30% during the first year. These results are comparable to those generally observed among white occidental populations. They confirm the limited incidence of hemorrhages (contrary to what has been observed in Japan), emphasize the important and often underestimated role of lacunar syndromes, and the seasonal variations specific to each mechanism.