With a view to elucidating some aspects of round-worm infection and mass anthelmintic treatment in rural area, the author conducted an investigation of all the pupils of elementary and lower secondary schools in Shirahama-machi, Chiba Prefecture (5 schools, 2, 180 pupils). Monthly stool examination was carried out from Feb. 1957 to Mar. 1959, together with more than ten times of mass anthelmintic treatment in between. Thus, such facts as transition of positive reaction rates, speed of infection or how soon infection occurred, infection rates by seasons, relationship between consecutive anthelmintic treatment and rate of conversion to negative, and that between the frequency of treatment received and the cumulative rate of conversion to negative were obtained by schools. The results of study are as follows.
1. Althouge all the five schools belong to the same local administrative unit, machi, their initial positive rates showed a considerable variance, the highest rate being 58.5% and the lowest 12.5%.
2. The infection speed by schools was high for the groups with high initial positive rates, in regard to both new- and re-infection. The cumulative positive rates by schools at the end of the first year ranged between 63% and 26%. The infection speed for the re-infection cases showed a higher value than that for the new infection cases.
3. Infection rates by seasons inclined to be lower in June and January, and higher in November and February to May inclusive.
4. Both Macnin and Nemacin, when employed repeatedly in consecutive treatment, appeared to cause a gradual decline in the rate of conversion to negative;and the severer the degree of infection, the more difficult it was to attain conversion to negative, it seemed.
5. The cumulative rates of conversion to negative obtained were: 78.6-94.3% by two courses of anthelmintic treatment, and 94.7-100% by four courses.
6. As for the transition of positive reaction rates, the rates became lower in the second year as compared with the first year of anthelmintic treatment program. However, in spite of the consecutive treatment given almost monthly, it would not fall beyond a certain level but rather showed a kind of side crawls (5-15% on an average).
On the basis of the above-mentioned findings it was concluded that, taking into consideration the infection speed, treatment should be give more frequently to the groups with high initial positive rates and to the reinfection groups, although it is generally said that mass treatment should be given 2 or 3 times a year.
Furthermore, it was considered that it would be difficult to attain the objective of the extermination of round-worms only by means of mass anthelmintic treatment, and that the treatment should be executed side by side with such measures for preventing reinfection as hygienic education, environmental improvement, and esp. the hygienic disposal of human waste, etc.

Real-world data (RWD), such as a health insurance claim database and electronic medical record database, which records daily medical care information, is one of the most important data sources in pharmacoepidemiological studies. In Japan, a post-marketing database study has been added as a new category of post-marketing surveillance since April 2018. It is expected that the post-marketing database studies will be conducted in the actual risk management plan； however, at this point, few have reached the stage of publishing study results. On the other hand, overseas, many database research results using RWD have already been reported. Although it is necessary to keep in mind that there are differences in the characteristics of the databases (e.g., data contents, structures) and the differences in the medical environment between overseas and Japan, we think that critically reviewing these reports has a reference value for the planning, execution, and interpretation of the results in performing post-marketing database studies in Japan. The purpose of this report is to examine the characteristics and cautions of the database studies through a critical review of published articles of overseas RWD studies and to make recommendations that are useful for conducting post-marketing database studies in Japan. We hope this article will become a help in the planning and implementation of future post-marketing database studies.