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--- howtos:workwithdata:project_into_history [2009/10/14 21:22]
shona.weldon
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-====== Project Data Back Into History ======
-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
-===== Linear Trend =====
-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.
-<code>
-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]
-</code>
-The main danger here is that your linear trend variable, ratioFamPerPerPH[cr,t19101977], goes negative

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