Final answer:
Most nursing research in the early twentieth century focused on the effects of sanitation on health promotion and disease prevention, which played a critical role in increasing life expectancy and advancing public health.
Step-by-step explanation:
The focus of most nursing research studies conducted during the first half of the twentieth century revolved around the effects of sanitation on health promotion and disease prevention. The era was marked by key developments in public health and medicine, which included understanding the transmission of diseases and implementing systems for clean water and waste disposal. These interventions, along with the advent of immunizations and antibiotics, played a significant role in raising life expectancy and enhancing public health. Nursing research of the time likely reflected these concerns, focusing on practical measures for disease prevention such as improving sanitation and hygiene practices, rather than on educational methods, epidemiological risk identification, or dissemination of research findings.