QuoteOriginally posted by: CuchulainnQuoteOriginally posted by: Traden4AlphaQuoteOriginally posted by: CuchulainnQuoteOriginally posted by: Traden4AlphaQuoteOriginally posted by: CuchulainnIs overshoot an issue?Indeed! Worse than that, this interpolation gets the wrong answer -- the sum of the 3 interpolated months (i.e., the integral of the interpolating curve) should equal the quarterly values. (Note even linear interpolation gets this wrong)One could convert the growth rates into levels and then interpolate.I don't see how integrals are involved here. Is some kind of preprocessing needed? Missing something?The data is quarterly growth data, right? If depends on whether it's an annualized rate, quarterly growth, or year-on-year. If it's an annualized rate, then you are right. If it's quarterly growth, then the monthly growth must combine to be the quarterly figure. And if it's YoY, then the constraint is even messier.So, what is the root problem? GIGO?It's not GIGO. It's an extension of what you said in your first reply. The choice of a interpolating function depends on the structural properties of the system. For example, should first derivatives, second derivatives, .... be continuous across the data points? In the case of some kinds of growth data, the constraint is on the first integral of the data.