Source: DiCecio, Riccardo
Relative price of investment goods is calculated as investment deflator (https://fred.stlouisfed.org/series/INVDEF) divided by consumption deflator (https://fred.stlouisfed.org/series/CONSDEF).
For additional details see: R. DiCecio (2009). "Sticky wages and sectoral labor comovement," Journal of Economic Dynamics and Control, 33(3): 538-53. (http://dx.doi.org/10.1016/j.jedc.2008.08.003).
DiCecio, Riccardo, Relative Price of Investment Goods [PIRIC], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/PIRIC, October 21, 2017.