A recent headline read, "School Lunches Make Kids Fat." The article cited a study comparing children at two schools. The school with the lower obesity rate only had a few children who ate lunch provided by the school. The other school had a greater percentage of obese children and most of these children ate school lunches. No mention was made that many of the children who eat school lunches are low-income children who receive free lunches. Poverty is a major factor in obesity. Which validity is threatened by the potential confound, poverty?