Final answer:
The Clothing Company used A/B testing to compare the effectiveness of two different email subject lines on customer click-through rates.
Step-by-step explanation:
In the scenario provided, the Clothing Company compared the effect of two different email subject lines on customer click-through rates by sending one version to a set of customers and another version to another set. This method is known as A/B testing, which is a type of experimental research that involves comparing two or more versions of a variable to see which one performs better in a controlled environment. In A/B testing, two variants (A and B) are tested against each other to determine which is more effective. It is a widely used technique in marketing to improve the effectiveness of marketing materials, including email campaigns.