All expository writing prompts give students a reason to write.
However, a prompt that does not contain enough information so
students can respond appropriately is worth than worthless. A
poor prompt ........
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Creates frustration for students.
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Damages your credibility as a writing teacher.
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Makes additional work for you as students keep interrupting
you to get more information about the assignment.
Who needs that grief?
A good prompt from Nation's Report Card
After each National Assessment of Educational Progress (better
known as Nations Report Card), the federal government posts
sample writing prompts from the test on its website. Ill
reproduce one here with my annotations so you can see that the
questions follow the guidelines
for a good writing prompt.
The first sample prompt I chose is a medium difficulty expository
writing prompt for eighth graders.

What is good about this expository writing prompt?
It provides context.
The topic is stated clearly.
The range of acceptable responses is specified.
The prompt says who the audience is.
The directions are clear.
Take a look at the other
sample expository writing prompts from the Nations Report
Card tests. There are prompts for use at eighth grade and
twelfth grade. You can use the questions as models or actually
use them in your classes as long as you include the source copyright
information. You would want to do that even if it were not required
because it makes the assignment look more serious.
A badly-written prompt
A student posting on Yahoo Answers asked for help deciphering
a writing assignment. Im guessing the assignment was for
a college class.
Every person who responded to the students plea for help
had the same reaction: the assignment was not clearly written. Here's
what the prompt said:

Whats wrong with this as an expository writing prompt?
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Theres no context indicated.
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The topic is vague.
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Multiple assertions about the topic are required making
formulation of a thesis extremely difficult.
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The point of the assignment is not indicated.
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No audience is specified.
- The format for the final paper is vaguely described.
Besides all that, the prompt is written as a single 90-word
sentence. Grammar-checking would have pointed out that the
sentence was far too long.
Give the instructor the benefit of the doubt: the typo might have
been created by the student.
The student who asked for help said shed tried chopping
the assignment apart and had checked the resources the teacher
listed, but still did not know what to do. Thats the nub
of the problem: the prompt does not give students help getting
started.
Ideally,
they also provide enough collateral materials that students can
do the entire assignment without running to you for help.