What difference does line want to convey " Proxies" and " Not as measure" of systematic risk

CFA Level I
#1

Security market indices are used as proxies for measuring market or systematic risk ,not as measures of systematic risk.

#2

Well, I suppose it comes down to the fundamental question of what is “the market portfolio” in the Capital Asset Pricing Model ?

In theory, it should represent ‘ALL investable assets’.

However, in practice there is no single market index that can perfectly represents all investable assets. So hence the the term “Proxy of Systematic Risk” rather than “Measures of Systematic Risk”.

#3

Thanks