Ensure that you're using the most up-to-date data available: Buy the 2017 Valuation Handbook - U.S. Guide to Cost of Capital + Quarterly PDF Updates together
The New Industry Standard in Business Valuation Reference Materials
2017 Valuation Handbook - U.S. Guide to Cost of Capital provides the key annual valuation data previously published in (i) the now discontinued Morningstar/Ibbotson SBBI Valuation Yearbook...