The arranging of experimental units or subjects into different groups so that the experimental units or subjects are similar to one another. this is done under the belief that the response to the treatment may potentially be different for the different groups.

Usually experiments are done by random assignment of the subjects or objects. This helps the study reduce bias and other potential threats to the credibility of the study. Thank you for your question. Please don't hesitate to ask in your queries. 

Since there is no true way to measure the money you spend on advertising with how much money it brings in to your business, there are some other ways you can measure your Ad effectiveness.  You can measure effectiveness by monitoring how your sales change when Ad's go out, new customers, traffic online and in the store and with, click-through rate method. 

The click-through rate method is an older method of seeing how many people click on you banner Ads on your website. The best way though, now-a-days, is to monitor how many people are on your website, how long they stay and, what they view when they're on there.