$.$ As you'll see in Question Two of this problem set, I've given you some real ETF data to work with, consisting of weekly returns going back to $2019.$ One of these ETFs is the Vanguard Total Stock Market $($VTI$),$ which tracks the overall U$.$S$.$ equity market. However, starting in $2019$ is far too short a time period to use historical data to get a meaningful estimate of what future market performance might be$.$

Suppose I instead gave you market data going back to $1926.$ In fact $-$ I do give you this in the F$-$F Research worksheet $($more on this in Question Two$).$

Or$,$ how about if I gave you data on U$.$S$.$ returns going back to $1880?$ Or British returns going back to the $1700\text{'}$s$?$

You could also take averages of these returns and use them to estimate future market performance.

What issues might you face when using historical data like this to try to say something about future returns?