2017年に￥1,582,858でSP500を買っていたら...

The S&P 500 is an index of 500 large US companies, weighted by float-adjusted market capitalisation and maintained by S&P Dow Jones Indices. It is the standard proxy for the US stock market and the benchmark most professional returns are measured against. Two things matter for reading it here. First, it is an index, not a security - you cannot buy it directly; investors use index funds and ETFs that track it, which carry fees. Second, the quoted index level is a price index: it excludes dividends. Since dividends have historically contributed a meaningful part of US equity returns, a price-only figure understates what a real index investor would have earned.

The index took its current 500-company form in 1957. Its history is best read through its drawdowns, because those are what an investor actually has to sit through: roughly half its value lost in the 2000-2002 dot-com unwind, close to 57 percent peak to trough in the 2007-2009 financial crisis, a fast 34 percent fall in the 2020 pandemic crash that recovered within months, and a decline of about a quarter through 2022. Recovery times have ranged from weeks to more than five years. Constituents change continuously as companies are added and removed, so the index is not a fixed basket - its long-run return reflects that ongoing turnover.

This is the control case. Every other scenario on the site is shown against it for a reason: a large multiple means much less if the broad market produced something similar over the same window. Running the index on its own answers the prior question - what simply being invested would have done.