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MATERIALS AND METHODS

As explained earlier, the material from the primary investigation carried out in 1958 was put aside for future use as the foundation of a prospective investigation for which no further contact between the investigator and the children was planned. In connection with the 1958 primary investigation, the parents and children were offered help and support but since then all the data needed for the prospective investigation came from the official registers kept by various authorities in the Swedish community. This information includes details of the civilian status of the individual, incomes, measures effectuated in accordance with social service regulations and pertaining to social assistance, child welfare and the care of inebriates, and notations referring to criminal offences and to physical and mental health. The initial purpose of the investigation was to elucidate the life situation of the individual after 15 years, but in the course of the work this period was extended to cover 20 years. As reported earlier, the reason for this was the results found in another longitudinal investigation (2,3) that was being run at the same time at this Departement. The collection of data began in the spring of 1973, i. e . 15 years after the first examination, and the intention was to elucidate the life situation of the child when it had reached the stage of a young adult by July 1, 1973. In addition to a summarized compilation that was put together after obtaining information from the Stockholm Municipal Social Welfare Register for 1974, collections were made of all the data for the first 15-year period and for the following 5-year period but no work was done on this information until the entire material had been collected. The work of compiling the data began on July 1, 1979 and covered the 20-year period from July 1, 1958-July 1, 1978. (The compilation of the data supplied by the National Health Insurance Register has also included the whole of 1979.) Mention has already been made of the report of the first 15-year follow-up period presented by the present author (4), and a comparison between these results and the results of the 20-year follow-up study will form the basis for a discussion of the methodology applicable to longitudinal studies in a forthcoming investigation.