Hey Kate,
I've not reviewed non-fiction as a habit. When I have, it was because I found the request on the 'please review' page. It was an opinion/lifestyle type of article, and before I did anything, I emailed the author and asked specific questions:
1. Who is your intended audience (age, demographic, etc)
2. If it would appear in professional print, where do you believe you'd like it to be (msn, GQ, NYT, buzzfeed, etc?)
I indicated my questions were not to be critical at that moment, but that I wanted to be able to at least find a few articles for comparison. IMO, there is no point in comparing apples to avocados. So I read some articles from the author's intended area and based my suggestions around that. And I learned in the process that lifestyle articles are a weird genre. You have to find a way to be different while being identical. It's so weird and I had never really noticed until I read several on the same topic back-to-back.
As far as fact checking, that can be really time consuming. I mentioned I generally do it purely as a curiosity, unless it's something I already know, then I find a link to further explain what I'm suggesting a correction for. I suppose if someone asked me to assist in playing devil's advocate with their facts, I would do it if I had the time to give it proper attention.
I don't know, maybe my reviews are overkill. I've had very kind and generous feedback on them, for the most part, which is why I've continued with the way I do them. The downside is I'm much slower and don't get many reviews out per month.
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