CASE: Hospitals' death rates are higher when nurses' education levels are lower, a study of surgical patients found. The study examined death rates at hospitals where the percentages of nurses with B.Sc. Degrees ranged from zero to 77%. Procedures at all hospitals included common operations, such as knee replacements, appendectomies and gall-bladder removals. Note: Nursing education may be a 2-3 year diploma or a four-year B.Sc. degree. The study found that hospitals where few nurses had B.Sc. degrees had death rates twice as high as hospitals where most nurses had B.Sc. degrees, and this was after taking into account other factors like surgeon expertise, severity of patients' conditions, and availability of technology. According to the researcher, better educated nurses tend to be more proficient in critical thinking and have more chances to develop skills that are important in life-threatening circumstances. The researcher noted that many procedures like the ones she studied are performed in hospitals every year and suggested that increasing education of nurses could yield a substantial number of preventable deaths.

QUESTION: Identify threats to the internal validity of this research due to the research design that was used. Explain how each threat applies to this specific research.