To Diversify Or Not to Diversify?

I often ponder about portfolio diversification in my mind.  

On one end of the spectrum, there are investors like Warren Buffett and Charlie Munger who believe in concentrating their funds on their best ideas:

"Once you are in the business of evaluating businesses, and you decide you are going to bring the effort and intensity and time involved to get that job done, I think that diversification is a terrible mistake... If you can identify six wonderful businesses, that is all the diversification you need and you can make a lot of money and I can guarantee you that going into a seventh one is going to rather than putting more money in your first one, it's got to be a terrible mistake.  Very few people have gotten rich on their seventh best idea." 

- Warren Buffet's 1998 Florida University address

"The idea that very smart people with investment skills should have hugely diversified portfolios is madness. It’s a very conventional madness. And it’s taught in all the business schools. But they’re wrong."

- Charlie Munger

On the other end, there are investors like Peter Lynch, who held thousands of positions at certain times in his Fidelity Magellan mutual fund.

Personally, I limit my watchlist to no more than 15 companies at a time.  But I wonder if I should cut it down to 10.  Or even 6 (best ideas) for that matter.

When you run a concentrated portfolio, other issues start to creep in.  For example, I find it challenging to build up a position in micro caps.  On some days, the bid-ask spread is so wide, that simply lifting the offer will cause a run up in the stock price.  Perhaps that is the reason why most investors stick to the bigger companies.

Then there is the nagging concern whether you have done all of the homework required - the effort, intensity and time involved as Buffett said - to invest in the right company?

I figure it all boils down to faith.  Faith in my own stock picking skills.

To diversify or not to diversify?  What are your thoughts?   

Would you concentrate your funds on your few best ideas, or spread it across multiple positions?

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