In an article covered in class titled "Indexing the Buffett way" covered in class, Warrant Buffett suggests "...investing in low-cost, market-tracking index funds rather than trying to pick individual stocks or using high-cost fund managers."

Explain what an index fund is. What are the reasons for even one of the most successful investors such as Buffett to be making this recommendation?

Also, if an individual investor wishes to replicate the performance of the index on his/her own, explain the two approaches (including advantages & disadvantages) of full replication & stratified sampling, that s/he can utilise.