A study (referred to in the October 12, 1999 issue of USA Today) reported the number of deaths by heart attack in each month over a 12-year period in New York and Boston. December and January were found to have substantially higher numbers of deaths than the other months. Researchers conjectured that the stress and overindulgence associated with the holiday season might explain the higher numbers of deaths in those months compared to the rest of the year.

(a) Identify a confounding variable that provides an alternative explanation for this result. Make sure you indicate how this confounding variable affects the two explanatory variable groups differently and how it is connected to the response variable. (c) A similar study conducted in Los Angeles also found more heart attack deaths in December and January than in other months. Identify a potential confounding variable from the New York and Boston studies that is controlled for in the Los Angeles study. Also identify a variable that is still potentially confounding in the Los Angeles study.

(d) Suggest a way of altering the study (e.g., another location to examine) that would help isolate the effect of the holiday season.