In an effort vs. In effort

anonymous Is it grammatically incorrect if I don't use 'an'?

Yes, it is.

Both are correct grammatically. However, it is much more clear to understand if an is used. You could also use the or even a. All would correct used in this sentence. When you write, as when you speak; be thinking how you can be as clear and descriptive as possible. Not only will it go a long way to keep the attention on you, but also to simply eliminate any misunderstanding. ??