A contributor's expertise and credentials should relate to your subject. A credible editor or author might:

• Hold a degree in this subject
• Study or do research on this topic
• Work in a related field
• Write about this topic regularly
• Have first-hand knowledge

A contributor's credentials and expertise are listed:

• In a section (e.g., "List of Contributors," "About the Authors," "About the Editors")
• In an editor's note before or after each short work