Martin Shkreli says something interesting in his 4th Finance Lesson video :
He explains that if you want to build a real portfolio with 20 stocks you need about 10,000 hours of work to do so – this translates in around 3 years of work. He estimates that if you really want to understand a company you have to do 100 hours of research on them – and that only 10% of the companies around are undervalued and worth buying so that you have to do 1,000h of work to find one company worth buying.
I would argue that you don’t have to do 100 hours of research per company if you focus on simpler ones. Also you can filter out a lot of companies in the beginning – thru a screener or following some investment-gurus. (But this clearly gives you no guarantee either)
But I like the point that investing is real/hard/painful work and not just some wealth creation machine were The Men Who Stare at Goats … forgive me … at Charts are making a lot of money.
You can think of Martin Shkreli what you want, you can disagree to him about investment subjects – but that guy really has some skills and he is sharing them with the world for free. I highly recommend watching his videos if you are interesting in the valuation of companies.