If one assesses this not in terms of data capture, quantitative analysis and other hard metrics, but from the user experience and sociological/psychological/behavioural direction, the implications are different.
For the reader (the end-user whose tasks, experience, and schematic understanding we're hoping to improve) believability is a matrix of authorial voice / tone, the perceived level of authority (based not on structural hierarchy but on skill or knowledge-based merit) of the writer and their data source(s), the relevance to the topic and / or meta topic being researched, and the clarity, structure and utility of presentation.
The term trustworthiness is germane to this discussion. Can I trust this source? Do they reliably present things in a way I can readily assess, apprehend and take action upon? Have they misled me previously? Does this information "feel" fast-but-loose, hastily-thrown-together, or worse yet apocryphal? Do I feel not only that my point of pain or confusion was clarified, but also reassured that my instructional source is of high value, that my time here was not squandered? do I feel my intelligence and competences have been understood and accounted for, or discounted out of hand?
I think this particular aspect of technical communications is now in a continually-escalating importance spiral due to the vast toxic seas of inaccurate, non-authoritative but easy-to-find rubbish purporting to be instructional content: especially in the peer-to-peer specific technique learning milieu.
The danger to user's perceptions of technical communications' value and believability is not necessarily in the direct instruction included in the specific skills tutorial or video, but rather the often deeply mistaken field-overview or meta comments thrown in by those peer coaches in passing: many users have taken something they "learned" from a trusted technique source and have constructed false understandings of far more critical and overarching ideas, only to later realise that these were deeply flawed. This leads to a certain degree of cynicism around instructional content.
How one goes about differentiating one's content as being of high trustworthiness seems critical to me; not only for actual acceptance of methods, modes and concepts, but to save our readers time and frustration, and to engage them further to improve both content and process.
I'm glad Yoel is putting time into researching this, and look forward to hearing about those results.