Standards of Evidence – who to believe

I posted this on Facebook, and asked people:

“How do you form robust arguments against these statements?”

No one did, maybe they didn’t want to, maybe they couldn’t. Sure, a bunch of people chimed in with their opinion but no more. A few said ‘studies show’ but what studies? I didn’t get involved but it’s a very valid question!

Our beliefs are powerful and as much as we may not like to admit it; we are easily influenced. Yes, even the ones who insist that they are not!

When I was making dietary modifications, I researched as many ways of eating as possible, even those I believed were not good for me. Why? Because maybe I was wrong in my beliefs. I mean, it’s possible. I didn’t even know where my beliefs came from, or when… and you probably don’t either!

It’s not meant as a dig, it’s genuine if someone hit you with quick-fire questions such as:


  • why do you believe it’s healthy?
  • who told you?
  • when did they you?
  • what research did you do?
  • did you read any conflicting research?

Most would struggle.

Look at the claims on the image of the steak. Do you believe any of them, some of them, none of them? Why, why not?

You see what I mean, refer back to the quick-fire questions above.


You have to look at the best available evidence, and it is highly unlikely that you authored that evidence, regardless of whether you believe it or not!


Let’s blast through some standards of evidence from the best, to the worst. Or more accurately, the most accurate to the least accurate on a balance of probability.


Level 1 Evidence

The highest standard of evidence. Evidence from a systematic review or meta-analysis of all relevant RCTs (randomised controlled trial)or evidence-based clinical practice guidelines based on systematic reviews of RCTs or three or more RCTs)


Level 2 Evidence

Obtained from at least one well-designed RCT (e.g. large multi-site rct).


Level 3 Evidence

Obtained from well-designed controlled trials without randomisation (i.e. quasi-experimental).


Level 4 Evidence


Well-designed case-control or cohort studies.

Level 5 Evidence


Systematic reviews of descriptive and qualitative studies (meta-synthesis).


Level 6 Evidence

A single descriptive or qualitative study.


Level 7 Evidence

The opinion of authorities and/or reports of expert committees. (Non governmental!)


Level 8 + 9 + 10 Evidence

YouTube videos, Charlatan, person you don’t know their professional background on Twitter, Facebook etc.