好問題嚟,你加個Tesla係會分散咗,但因為個idiosyncratic risk大到痴線->超級大嘅波動,導致加咗落去個投資組合之後個新嘅投資組合都會變得頗大波動,但個expected arithmetic return 唔應該有分別,所以個Sharpe Ratio會變低
加factor係首先假設你已經係嗰個Factor入邊係分散咗,所以只係得返systematic risk,第二 factor 嘅 expected arithmetic returns 會高過market 少少,咁回報高啲,波動同market factor差唔多,加埋imperfect correlation先有高啲Sharpe ratio
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