Why is it vital to establish a 'control group' in A/B testing?

Establishing a 'control group' in A/B testing is vital because it provides a baseline against which to measure the impact of the changes being tested. The control group receives the original version (version A), while the test group receives the modified version (version B). By comparing the performance of the two groups, you can isolate the effect of the changes and determine whether they are truly responsible for any observed differences. Without a control group, it's impossible to know whether the observed changes are due to the tested variable or to other external factors, such as seasonality, changes in market conditions, or unrelated marketing campaigns. The control group ensures that the A/B test results are reliable and valid.