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Comment by teej | Hacker News Reader

If you’re keeping a balanced portfolio of assets and the price of one goes up, you sell to rebalance, not buy.

Is that because the underlying indeces are defined as each asset being a fixed percentage of the whole? Like, hypothetically, the S&P 500 would define MSFT as 1% of the index so if MSFT goes up then the fund would have to sell it to keep it from being more than 1% of the total holdings?

For what it's worth, the S&P 500 uses float-adjusted capitalization weighting.

https://en.m.wikipedia.org/wiki/Capitalization-weighted_inde...

After reading more about stock index funds I'm more confused how their rebalancing works. I now understand that the S&P 500 index is basically the total market cap of the free float of the 500 companies combined. I just don't see why a tracking fund would have to rebalance at all. If you own the 500 stocks at the right proportion it should stay balanced as each company's market cap grows or shrinks relative to all the other companies. The only time you would have to rebalance is when companies are added or removed from the index.

It seems like you mostly have a good understanding of how it works. A tracking fund would have to rebalance in the advent of a company issuing or buying back shares, when there is a corporate action like a spinoff or merger, or when a company is added or removed from the index.

Additionally, you have the reality that every day, investors buy and sell that fund. The tracking fund, therefore, has to liquidate or buy new holdings to match the NAV (net asset value) as shares are created or redeemed. This is where tracking funds will vary in their performance as some ETF providers might be a lot worse at matching up inflows and outflows.

A market-cap weighted tracking fund still has to rebalance if one of the companies issues new shares, or buys some back, as well. But that's a relatively unusual situation (like dozens of events per year, not daily).

A couple of things:

1) A fair few of those 500 stocks are not very liquid, you'll lose a fortune chasing the shares.

2) Think about what happens if you want to provide exposure that isn't 1x the index.

3) What happens when there's a replacement?

4) What about things that aren't statically replicable?

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