> Take the s&p 500. The biggest constituent, Nvidia, accounts for 8% of its value. Imagine the $5.3trn chip-designer’s share price moves up or down by 3% (as has happened 30 times this year) and the other 499 stocks are flat.

That scenario (NVIDIA moves by 3% and no other stock moves) never happens. The market is dynamic, so all the stocks move at different magnitudes, volatility, frequency, etc. that is represented in the index.