Life-span of amalgam restorations in primary molars: some results and comments on statistical analyses

The life-span of amalgam restorations placed in primary molars by three general dental practitioners is investigated. Two methods of analysis have been carried out. In the analysis of "non-independent" restorations (i.e. using several restorations from each mouth), the median survival time was 52.8 months with an apparent standard error of 2.1 months. In the analysis of "independent" restorations (i.e. only using one restoration from each mouth), three random samples were chosen and their median survival times were 68.2 months (s.e. = 6.9), 60.5 months (s.e. = 6.9), and 56.8 months (s.e. = 3.7). The two methods are compared and discussed. It is concluded that the analysis of "independent" restorations should be the method of choice in studying the life-span of restorations.