Claude Hopkins Wrote In His Timeless Classic, Scientific Advertising:

“To properly understand advertising or to learn even its rudiments one must start with the right conception. Advertising is salesmanship. Its principles are the principles of salesmanship. Successes and failures in both lines are due to like causes. Thus every advertising question should be answered by the salesman's standards.

“Let us emphasize that point. The only purpose of advertising is to make sales. It is profitable or unprofitable according to its actual sales.
“It is not for general effect. It is not to keep your name before the people. It is not primarily to aid your other salesmen. Treat it as a salesman. Force it to justify itself. Compare it with other salesmen. Figure its cost and results. Accept no excuses which good salesmen do not make. Then you will not go far wrong.

“The difference is only in degrees. Advertising is multiplied salesmanship. It may appeal to thousands while the salesman talks to one. It involves a corresponding cost. Some people spend $10 per word on an average advertisement. Therefore every ad should be a a "super-salesman".

“A salesman's mistake may cost little. An advertiser’s mistake may cost a thousand times that much. Be more cautious, more exacting, therefore. A mediocre salesman may affect a small part of your trade. Mediocre advertising affects all of your trade.”