The Ibbotson-Chen model of Risk Premium

Tang estimates the expected inflation rate to be 2.6%. She expects real domestic growth to be 3.0%. Tang believes that the markets are currently overvalued by 3%. The yield on the market index is 1.7%, and the expected risk-free rate of return is 2.7%.

I understood every part of the solution the problem provided. However, why did they do the below for the P/E ratio? I actually did 1+0.03 to calculate the expected growth in the P/E

PEg = relative value changed due to changes in P/E ratio = −0.03

ERP= (1.026) × (1.030) × (0.97) − 1 + 0.017 − 0.027 = 0.015 = 1.50%

In the long-term, the market should be fairly valued (i.e. no misevaluation).

So, if the market is overvalued by 3% (now), then in the long-term, the market P/E should decline by 3% (at fair value), hence why you substitute \Delta P/E = -3\%