In this example we can show you how to make a subject more specific:
Subject (what, where and when): Obesity in the Netherlands in the 21st century.
The subject is still too broad and produces too many hits. Select an underlying subject and make the period more specific: Influence of food supply on obesity in the last 10 years.
The subject and location can be more specific: Influence of the food supply in Dutch school canteens on obesity over the past 10 years.
A very last specification of subject and category ultimately results in the following question: Over the past 10 years, what has been the influence of food supply in Dutch school canteens on obesity among higher education students?