如果有100块钱。是还贷款还是投资SP500?
基本假设:
1. 贷款利率3%
2. 个人所得税最高税率25%
第一种:还贷款
贷款节省:+3 ($100的利息是$3)
没有抵税了,损失-3*0.25=-0.75
所以总共赚3-0.75=+2.25
第二种:投资SP500
投资SP500 收益:假定是x
贷款利息付出:-3
贷款抵税赚:+0.75 (同第一种算法)
投资应交税额:-x*0.25
所以总共赚:x-3+0.75 -0.25x
那么,什么时候应该做第一种(还贷款):
就是当x-3+0.75-0.25x> 2.25
0.75x> 2.25 + 2.25
x > 6.
即,SP500要6% risk-free 回报才值得投。
Investopedia 上的一段话:
Estimates of the Equity Risk Premium
The majority of economists agree that the concept of an equity risk premium is valid: over the long term, markets compensate investors more for taking on the greater risk of investing in stocks. How exactly to calculate this premium is disputed. A survey of academic economists gives an average range of 3–3.5% for a 1-year horizon, and 5–5.5% for a 30-year horizon. CFOs, meanwhile, estimate the premium to be 5.6% over T-bills (U.S. government debt obligations with maturities of less than one year) and 3.8% over T-bonds (maturities of greater than 10 years).
所以,如果以30年计算,用5%,那么,SP500需要6+5=11% 才值得投。
这跟平常的结论大相径庭。
这个算法,在那些假定的前提下,有什么误差吗?
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