Product-market Fit
20VC: How To Scientifically Measure Product-Market Fit...with Daniel Er The Twenty Minute VC
https://www.viable.fit/
Anna-Lisa: Sometimes some hustle is needed. It’s the Reid Hoffman quote, “if you are not embarrassed by the first version of your product, you’ve launched too late.”
That said, you need to know when to pause and reflect on what your feedback loops are telling you, and whether you are prioritising correctly
What button on my MVP landing page is most clicked? When I moved my checkout button did it increase or decrease conversion? Who are the customers that are getting all the way through the user journey?
Start-ups in our portfolio have benefited from working with people with learning difficulties
Feedback loops mean that you are never the ones to disappoint
... better than 1000 words
compare with product-market fit 
and founder-market fit 
The only thing that matters
Eric Ries Assessing fit with the Wisdom of Crowds
Lecture 14 - How to Operate (Keith Rabois)
https://leanstack.com/leancanvas
example - https://www.scribd.com/doc/67465758/Sean-Parker-s-Email-to-Spotify-s-Daniel-Ek