A source is said to be credible when it is reliable consistently.
Step-by-step explanation:
A source becomes credible when the source is reliable constantly , which means it is providing correct information constantly . When the information provided becomes true a trust factor starts working for that source and the receiver starts believing the information provided in that source.
In case of academic writing a source is said to be credible when it is providing unbiased information and along with proper evidence. While writing a research paper we always depend upon a credible source.