When it comes to fund-raising letters, longer is usually better, according to Jeff Brooks, a consultant at Merkle, a Seattle direct-marketing company, and author of Donor Power Blog.

Mr. Brooks says that over the past five years of testing various direct mail formats he has only once seen a shorter letter get better results than a longer one.

That’s because the type of donors who respond best to letters are a self-selected bunch who prefer to get their information through reading, says Mr. Brooks, adding that the content’s relevance to the donor is a far more important factor in an appeal’s success than its length.

“Your donors may be different. You may have tested into more success with shorter letters,” he writes. “But if you’re relying on short letters because you personally don’t have time to read longer letters, you’re probably making a mistake.”

Do you agree? Tell us what has worked best for your organization.