How Audience and Purpose Affect Focus
All readers have expectations. They assume what they read will follow a logical order and support a main idea. For instance, an essay arguing for a second skating rink for hockey players should not present cost figures on how expensive new uniforms have become.
Your audience and writing purpose will help you determine your focus. While it may seem obvious to include certain details, your audience may require specific information. Further, why you are writing will also affect what information you present.