You could narrow the topic of marketing to special groups by identifying specific special groups as a sub-topic.

Marketing
an age group		e.g. marketing to adolescents
a location		e.g. marketing in Japan
an additional topic		e.g. marketing in magazines
a population		e.g. marketing to minorities or women

It is helpful to write out your topic as a sentence or a question.

Let's say the question that most interests you is:

How do advertisers market their products to women?