Hi. My question is from Stata (15.1), but also requires some statistical knowledge, which is why I am calling on you.
I'm doing a homework, and I have three different models.
1: ln (y) = b0 + b1 * X + error
2: y = b0 + b1 * ln (x) + error
3: ln (y) = b0 + b1 * ln (x) + error

I have a database with the variables x and y. I have to tell among the 3 models, which assumes a decreasing marginal relationship between Y and X.
How can I do with Stata?

When I use:
Code:
* 1: log-lin
gen ln_y = ln (x)
reg ln_y x
mfx
* 2: lin-log
gen ln_x = ln (x)
reg y ln_x
mfx
* 3: log
reg ln_y ln_x
mfx
At the mfx command, I always get postives results >0 (but I should have a result <0 in order to have a decreasing marginal relationship, isnt it ?).

Even when I derive the functions, I get positive results. Do you have an idea that can prevent from failing this assignement ?




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