lookup table

[sani]

hello
please help me
My lookup function f multiplies only one constant in growth rate the variable

[Administrator]

Given the numbers in the lookup, I think you need
growth rate = f ( time )

Is time (or year) the x axis in your lookup?

[sani]

Yes x-axis is time and y-axis is GDP growth rate

[tomfid]

To suppress units warnings you can make the x-input Time/Unit Time (where Unit time = 1 with whatever time unit you need). Then:

GDP Growth Rate = GDP Growth Lookup( Time/Unit Year ) ~ fraction/year
GDP Growth Lookup ... {table}
Unit Year = 1 ~ year

[sani]

Hello
thanks for your help.
I modify growth rate = f ( time ) and I change units, but I still have a problem!

[Administrator]

Did you also change the time bounds of your model so it runs from 2016 onwards?

[LAUJJL]

Hi

A growth rate that dépends on time, looks Strange.

generally the growth rate dépends on the level, like a bank account with interests.

Regards.

JJ

[sani]

Did you also change the time bounds of your model so it runs from 2016 onwards?

excuse me, How should I do this?

[tomfid]

JJ - I think it's a terminology issue. In my suggested equations, "growth rate" refers to the fractional rate of increase, in units of fraction/year, as distinct from the actual growth that occurs, in $/year/year. So, the full equations might be:

Code: Select all

GDP = INTEG( GDP Growth, Initial GDP ) ~ $/year
GDP Growth = GDP * GDP Growth Rate ~ $/year/year
GDP Growth Rate = GDP Growth Lookup( Time/Unit Year ) ~ fraction/year
GDP Growth Lookup ... {table}
Unit Year = 1 ~ year

[sani]

Thank you so much for your valuable guides.

[LAUJJL]

Hi Tom

Your équations are mathematically correct (see model joined) but I was saying that a growth rate depending on time was economically not very realistic.

The growth rate is realistically more a constant like an interest rate , one of the run joined than growing linearly like the second run joined.

Regards.

JJ

[sani]

Thank you for your all your help.

[tomfid]

Indeed you are correct. A growth rate dependent on time might be convenient if you just want a GDP scenario as an input to a model of something else, but it isn't much if you're interested in modeling the economy itself.

[LAUJJL]

Making a variable time dependant, is like inputing this variable as an external data.

[sani]

Despite all the changes and corrections, when I put the actual data about GDP growth along years in the lookup table, it seems that the output is still wrong.
GDP starts from zero point instead of 393.4 billion dollars!!

[LAUJJL]

Hi

Here is the model corrected, with a unit correction (the model has strictest unit testing set on) and the initial GDP set outside the GDP equation (best practice).

the llokup now starts from year 1 (2017) to year 2 (2018.

The model is in published format (vpm) downloadable with any Vensim version with the run growth 4 included.

Use the table tool to observe the GDP and not the Graph one where the the GDP looks like starting from 0.

Regards.

JJ

[sani]

Thank you so much.