Often you have a variable, say a demand, for which you have measured data over some historical period up to your simulation period. You may need to project that demand back into history.

The best way is to get a ratio of that measured value to some driver like population over the known period. Assuming you have the driver back into “pre-history”, you can then project the ratio back in several ways as described below

The simplest way is to linear trend it back into history and then multiply it against the known driver in “pre-history”. For example getting families in from 1910 to 1977 when we know families and population over historic time th.

local ratioFamPerPer[cr,th] = familiesTot[cr,th] / CTotPop[cr,th]
local ratioFamPerPerPH[cr,t19101977] = lintrend (ratioFamPerPer[cr,th]; time=1910)
familiesPH[cr,t19101977] = extract (CEEFpop[cr,t18511990]; time:1910..1977) * ratioFamPerPerPH[cr,t19101977]

The main danger here is that your linear trend variable, ratioFamPerPerPH[cr,t19101977], goes negative