&quot;You don’t need matrix analysis to know that if you tell everyone to wash their hands, infections will go down.&quot;<p>No, but you will need a well designed study or two, maybe a few accidental experiments (situations where a single variable is changed among two randomly selected groups of people), to tell if it&#x27;s actually true.<p>And it may be that your hypothesis is incorrect. It may be that telling everyone to wash their hands actually makes infections go up in some specific contexts. I can imagine a couple scenarios where that might be the case. One is the false sense of security of the washing hands message leads to a slightly sloppier application of other protocols such as sterilization of tools or cleaning furniture. Or the means of spreading the message its self, say on plastic posters, could be a breeding ground of airborn bacteria. Sure, these are highly improbable, and there&#x27;s even more I can&#x27;t imagine, but I wanted to point out that the original hypothesis isn&#x27;t as airtight as the author intended... if taken literally.