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====== Using a relative propensity ======
When you have an equation with more unknowns that constraints and you want to add some assumptions to solve it you may add a relative propensity calculation.  Best served by an example.  

Given:
   households[uss,usdt,th]
   population[uss,th]

Want to get:
   peoplePerHousehold[uss,usdt,th]

constraints:
   population[uss,th] = sum (households[uss,usdt,th] * peoplePerHousehold[uss,usdt,th]; dim=usdt)

makeup a relative propensity:
   relPopPersPerHH[usdt] = where each value is related to one of the entries 

Finally the math to calculate peoplePerHousehold[uss,usdt,th] is:

    !Equation A:
    local peoplePerHouseholdEst[uss,usdt,th] = households[uss,usdt,th] * peoplePerHousehold[usdt]
    !Equation B:
    peoplePerHousehold[uss,usdt,th] = population[uss,th] / peoplePerHouseholdEst[uss,usdt,th] * relPopPersPerHH[usdt]

===== Check your above equation through symbol substitution algebra =====
You know your identity :
    population[uss,th] = households[uss,usdt,th] * peoplePerHousehold[uss,usdt,th]
substituting from your final math Equation B for personPerHousehold:
    population[uss,th] = households[uss,usdt,th] * population[uss,th] / peoplePerHouseholdEst[uss,usdt,th] * relPopPersPerHH[usdt]
reordering:
    population[uss,th] = households[uss,usdt,th] * relPopPersPerHH[usdt] * population[uss,th] / peoplePerHouseholdEst[uss,usdt,th]
Then notice the substitution can be done for Equation A from the final math above:
    population[uss,th] = peoplePerHouseholdEst[uss,usdt,th] * population[uss,th] / peoplePerHouseholdEst[uss,usdt,th]
And the estimates cancel so you have an identity and you are comfortable your math worked
    population[uss,th] = population[uss,th]
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