Ask the writer to write about a subject he knows well: himself. This will provide a topic that he is comfortable writing about. The diagnostic essay will then be judged on his writing style instead of his ability to research a topic. Information to include in the essay is where he is from, some of his hobbies and where he works if he has a job. He could also write about his favorite sports teams.
Another possible topic for a diagnostic essay is a review of a business or a product. Everyone develops opinions of products or services she receives as a customer. In the diagnostic essay, ask her to evaluate the product or service as if she is writing an article for the local newspaper. Once again, this gives the writer a chance to write about something she is comfortable with, without having to do additional research on the product itself before writing the diagnostic essay.
Write a diagnostic essay about a favorite vacation spot. The writer should include information about where it is located and what makes it his favorite vacation location. The diagnostic essay should provide details that support this assertion, such as some of his favorite restaurants at the vacation location or some of his favorite activities to do while on vacation there. Include recommendations for the best time of year to visit that location and tips such as setting a budget for the trip.
Instead of introducing herself in the diagnostic essay, ask her to write about her family members. She should introduce the family members and include information such as what town they live in and some of their favorite hobbies. Include information about the relationship she has with those family members and what makes those bonds between her and the family members strong. Once again, providing a subject that doesn't require research will allow you to analyze her writing skills instead of the research needed to write the paper.