If you needed an expert´s opinion on a given topic when assembling an argument of any form, you should look for a testimony. This refers to an educated and coherent opinion that an expert delivers in a personalized manner about a given topic. Since a testimony is an opinion, it can only hold values of reliability if it indeed comes from an expert in the topic at hand. Testimonies as such can provide great insights into a problem and are very valuable as supporting material.
