In other words, the volatility in the total PCE price index makes it harder to discern trends (longer-lasting movements) from transitory movements. In addition, food and energy prices often depend on factors that are beyond the  Note that the core indexes are defined somewhat differently. Both core indexes exclude all energy prices.
At this level, conventional monetary policy can no longer be used to stimulate the economy – a risk that was brought into stark relief during the recent crisis. The lower the inflation target, the lower the “equilibrium” nominal policy interest rate will be, and thus the greater the likelihood that shocks will push interest rates to the ZLB during “normal” times.