Consider the following comparisonsWhich is safe from confounding, which appears contaminated by confounding? Identify possible confounding factors.

1. A comparison of earnings between employees who have college degrees and employees who don't have college degrees within the same firm.
2. A comparison of the frequency of dental visits between patients who have dental insurance and patients who do not have dental insurance.
3. A comparison of last year's revenues between companies that showed ads during the Super Bowl and companies that did not show ads during the Super Bowl.
4. A comparison of online vs. offline sale events using average daily sales during the sale events in a physical store vs. sales from online orders in the same period.