Best fit was
obtained when choosing distinct sets of values of the constants A, B and C for each of the three periods 1958 to 1980;
1981 to 1999; and 2000 to 2011. These changes could have resulted from the impact of the oil shocks in the 1970s and
very high interest rates in the 1980s, which perhaps heralded changes to the index formula in 1982 and 2002. The
error was within 3% in any year from 1958 to 2011, and within 1% from 2004 to 2011 after readjusting the weighting of
the price of oil. The correlation was applied to forecast the CEPCI under different scenarios modelled by the Energy
Information Administration or predicted from oil futures contracts.