If you know which stock will give the best return, you would be an idiot not to invest all your money into this one stock. The reality is that we cannot see the future so we hedge buy spreading our investments to several stocks.

But this works only if we have a diversified portfolio. https://www.i4value.asia/2022/01/how-to-construct-winning-stock.html#more

The stocks in the portfolio are not correlated to one another. How do you select the stocks to achieve the diversity?

We know that if you have 2 bank stocks such as Bursa Affin bank and Maybank in the portfolio, it is not a diversified one. What if we have Affin and UOA Dev a Bursa property company? Will we then have a diversified portfolio?
https://i.postimg.cc/Bbt4Wnnd/Affin-vs-UOA.png

The chart shows the ROE trends of Affin and UOA over the past 7 years. There is a 81% correlation between then. This mean if statistically that changes in Affin ROE can explain 2/3 of the changes in UOA ROE. This is significant

So is having Affin and UOA meant that it is not a diversified portfolio?