Studies made by Kate as a university student have shown that people over age 60 who donate blood twice a year are less likely to die from a heart disease during the following year than those who do not donate. However, recent research has shown that people over age 60 who donate blood twice a year are also less likely to die from any cause during the following year than those who don't donate. Are there any confounding variables that make it difficult to justify a cause-and-effect conclusion