Where should you focus your investment effort?

I started working around 2002 and started investing with almost all stocks in the earlier years.

1. earlier contribution has a lower cap. I recall $12K for my first year for 401a. 
2. started 457 plan in 2008. same limit as 401a. 
3. school typically matches 7-10%. some as high as 12-14%. when you add all up, it can be 45-50K in total contribution per year per person nowadays. 
4. DCA leads to lower overall cost and the base that caught the up trend since 2009 was far bigger than the base suffered DD b/w 07 and 09. 
5. Overall gain on capital contribution is about 70% overall. Really no need to go all stocks to achieve similar return.  

As I said before, the best investment for young people is their career. Before you have 1M in networth, just max out retirement contribution in SP500 and do not look at it. spending too much time on investing does not pay off. 

Human capital (rougly speaking the PV of lifetime earnings) grows at a higher linear rate initially, but does not compound exponentially. You will be lucky if it even grows in real term at all once you hit mid- to late 40s. Capital returns however compound exponentially. So once your capital base reaches a large enough level, you are better of focusing on capital investment, instead of human capital/career investment. 

Trying to make a killer trade / investment in order to become financially secure and retire early, for most of us, is just day dreaming. Money is earned and accumulated one $ at a time.

Just my 2c