Hi,
I am running a linear regression in Stata 17, using the regress command. Based on theory I suspect that the relationship between my independent and dependent variable might be non-linear, however, my sample is a bit different from previous samples, so this is not a given. Thus, I decided to chech whether I should include a quadratic term in my model, but I am having some trouble interpreting the results/deciding whether or not to leave the quadratic term in.
This is a cross sectional data set, and my dependent variable is funciton and my independent variable is strength.
As you see, the quadratic term is not significant here, so my first thought was that I should drop the quadratic term. However, when I graph the data, I still think it looks more non-linear... So I ran a margins plot. The range of my strength variable is 0.85 - 6.3
The result was the attached graph, and I would say that this looks quadratic/non-linear?
I did look at the axis of symmetry using nlcom returning the value of 5.63, which is within the range (0.85-6.3), but very close to the upper boundary.
All of this makes me confused. How do I interpret this? Should I leave the quadratic term in the model, and in that case, how do I interpret/expain that non-significant term? Could it be due to lack of statistical power when I include the quadratic term? It might even be that I have done something wrong here, this is my first time running a regression using a quadratic term. And I also applogize in advance if this post is difficult to read or understand, it is my first time posting in the forume as well.
Best,
Hilde
I am running a linear regression in Stata 17, using the regress command. Based on theory I suspect that the relationship between my independent and dependent variable might be non-linear, however, my sample is a bit different from previous samples, so this is not a given. Thus, I decided to chech whether I should include a quadratic term in my model, but I am having some trouble interpreting the results/deciding whether or not to leave the quadratic term in.
This is a cross sectional data set, and my dependent variable is funciton and my independent variable is strength.
Code:
regress function c.strength##c.strength i.sex
---------------------------------------------------------------------------------------
function | Coefficient Std. err. t P>|t| [95% conf. interval]
----------------------+----------------------------------------------------------------
strength | .2311146 .0905668 2.55 0.012 .0513168 .4109124
|
c.strength#c.strength | -.0205282 .014225 -1.44 0.152 -.0487683 .007712
|
sex |
female | .1430361 .0463664 3.08 0.003 .0509871 .235085
_cons | .2134218 .1359994 1.57 0.120 -.0565712 .4834148
---------------------------------------------------------------------------------------
Code:
margins , at (strength = (0.85 (0.2) 6.3)) marginsplot
I did look at the axis of symmetry using nlcom returning the value of 5.63, which is within the range (0.85-6.3), but very close to the upper boundary.
Code:
nlcom -_b[strength]/(2*_b[c.strength#c.strength])
_nl_1: -_b[strength]/(2*_b[c.strength#c.strength])
------------------------------------------------------------------------------
function | Coefficient Std. err. z P>|z| [95% conf. interval]
-------------+----------------------------------------------------------------
_nl_1 | 5.629207 1.861501 3.02 0.002 1.980733 9.277682
------------------------------------------------------------------------------
All of this makes me confused. How do I interpret this? Should I leave the quadratic term in the model, and in that case, how do I interpret/expain that non-significant term? Could it be due to lack of statistical power when I include the quadratic term? It might even be that I have done something wrong here, this is my first time running a regression using a quadratic term. And I also applogize in advance if this post is difficult to read or understand, it is my first time posting in the forume as well.
Best,
Hilde
