the owner of a chain of supermarkets notices that there was a positive correlation between the sales of beer and the sales of ice cream over the course of the previous year. during seasons when sales of beer were above average, sales of ice cream also tended to be above average. likewise, during seasons when sales of beer were below average, sales of ice cream also tended to be below average. the owner is curious whether the observed association is due to a cause-and-effect relationship between eating ice cream and desiring beer. to investigate this, the owner should use:
