Which of the following is regarded as a best practice for creating a campaign experiment?

Focus each test on one variable at a time and use different tests to assess the effects of more than one change.
After an experiment has ended, examine performance over a timeframe that includes a ramp-up period.
Select two or three metrics to assess campaign performance and determine the winner of a test.
Use a different approval process for new ads during experiments so that experiments can be conducted more expeditiously.

The correct answer is:

Focus each test on one variable at a time and use different tests to assess the effects of more than one change.