William Sutherland writing in Nature 14^th November vol 503 p 167 talks of Review by quality not quantity for better policy.

Society  increasingly demands that policies are based on scientific evidence. Yet he claims Policy makers rarely refer to the journal papers- basing their missives on summaries, reviews and assessments. Such assessments can be deeply influential but fallible.

Once codified it is difficult to change.

I seem to remember that it takes a short time to get a “fact” into text books and 10-20 years to get it out.

Sometimes one wonders if these providers of advise live in the real world. It is suggested for example that men can reduce the chance of a stroke by walking 12 or more hours a week. Hardly practical for many. What is the reduction in risk for such exercise?