Technological progress and advances in health research have led to an increase in human life expectancy. The decrease in mortality rates associated with improvements in basic sanitation is also a factor that results in an increasingly significant number of elderly individuals in the population and leads to a process of rapid and intense population ageing. As ranked by Niessen, Jones, Zochi and Gurian, individuals between 65 and 74 years old are considered "young elderly". These individuals are relatively healthy and active and are also known as senior citizens. Individuals between 75 and 84 years of age are the "middle aged", and this group includes those who are healthy and active to those who live with chronic diseases. People older than 85 years are also known as "older than the elderly." According to Mello, after years of living with chronic diseases, this group is psychologically more fragile. This group is the one that is growing the most rapidly in the elderly population.