What you are reading right now is a blog. It’s written and posted by me, because I want to. I get no financial remuneration for writing it. I don’t have to meet anyone’s criteria in order to post it. Not only I don’t have an employer or publisher, but I’m not even constrained by having to please an audience. If people won’t like it, they won’t read it, but I won’t lose anything by it. Provided I don’t break any laws (libel, incitement to violence, etc.), I can post whatever I want. This means that I can write openly and honestly, however controversial my opinions may be. It also means that I could write total bullshit; there is no quality control. I could be biased. I could be insane. I could be trolling.
Some blogs are monetized, often via advertising. This means that they still have no quality control, and are biased towards writing whatever may attract large audiences.
This is a newspaper article. It’s written by a journalist. Journalists are professionals; however, many of them are professionals at journalism. If they write an article about a specific subject, it doesn’t necessarily mean that they are experts on that particular subject.
Newspaper articles are subjected to a review process. On the one hand, this can ensure that journalists don’t write total dross. On the other hand, some publications consist almost solely of dross: spin, exaggerations, exposés of scandals that never took place, etc.. Shocking stuff sells, and newspapers need an audience. They make their money not just by selling the paper to the audience, but by selling the audience to the advertisers. The selection process does not necessarily focus on accuracy.
This is a scholarly article. It is written by an expert in that particular field, and peer-reviewed by other experts. This ought to ensure the highest possible level of quality control, if only scientists weren’t a. humans and b. paid for their work. Scholarly articles are often written in ways completely impenetrable to laypeople, making them hard to decipher, let alone critically evaluate them.
This… I don’t know what to call this. To call it “popular science” seems an insult both to science and to the population. Some publications produce allegedly scientific articles for laypeople. Most of them neglect to give sufficient information about the research behind the articles. Looking up this information can be both interesting and incredibly depressing, as many of the projects are blatantly statistically invalid, often to minuscule sample sizes. Then again, these publications have to produce articles regardless of whether any ground-breaking discoveries have been made or not.
This is a book. It’s written by a writer and published by a publisher. Contrary to public opinion, the underlying purpose of publishing isn’t the edification of all humankind. The publishing business is precisely that: a business. Books are selected by publishers based on whether they are likely to sell.
This is a self-published book – you can tell that because there is no mention of a publisher on the copyright page. Anyone can write and publish anything they want; for digital books, they don’t even have to pay for that privilege. A self-published book may or may not have been peer-reviewed, edited, or even proof-read. (As it happens, this one has been checked extensively; it’s my book and I wasn’t going to put my name on anything less than the best I could produce.)
The bottom line is: not all sources are equivalent, and all sources have their pros and cons. These needs to be taken into account when evaluating information, and all information should be evaluated.