When I use the eyex option of margins, what is it actually
computing and how does it relate to the coefficients of the loglinear model?
| Title |
|
Obtaining elasticities for independent variables |
| Author |
May Boggess, StataCorp
Kristin MacDonald, StataCorp
|
The eyex() option causes
margins to compute d(log f)/d(log x), where f is the prediction function
specified in the predict() option of margins or, if none was specified, the
default prediction option for the preceding estimation command.
The elasticity d(log f)/d(log x) can be calculated easily from the marginal
effect df/dx by using the chain rule. This gives the formula
d(log f) d(log f) dx_i
--------- = ---------- * ----
d(log x_i) dx_i d(log x_i)
Because d(log x_i)/dx_i = 1/x_i, we have
d(log f) d(log f) x_i d(log f) df x_i df
--------- = ---------- * ---- = x_i * --------- * ----- = ----- * ----
d(log x_i) dx_i 1 df dx_i f dx_i
where x_i is the ith independent variable in the regression. By default,
margins evaluates this for each observation and reports the average
of the elasticities. We can use the atmeans option to evaluate
this at the mean of the independent variables or the at() option to
specify specific values of the independent variables. If the
predicted value is negative, the elasticities cannot be computed because we
cannot take the log of a negative number.
We can verify that the above formula works. In this example, we will
calculate the elasticities at the means of the independent variables.
. sysuse auto, clear
(1978 automobile data)
. regress mpg weight length
| Source | | SS df MS | Number of obs = 74 |
| | | | F(2, 71) = 69.34 |
| Model | | 1616.08062 2 808.040312 | Prob > F = 0.0000 |
| Residual | | 827.378835 71 11.653223 | R-squared = 0.6614 |
| | | | Adj R-squared = 0.6519 |
| Total | | 2443.45946 73 33.4720474 | Root MSE = 3.4137 |
|
| mpg | | Coefficient Std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | -.0038515 .001586 -2.43 0.018 -.0070138 -.0006891 |
| length | | -.0795935 .0553577 -1.44 0.155 -.1899736 .0307867 |
| _cons | | 47.88487 6.08787 7.87 0.000 35.746 60.02374 |
|
.
summarize weight
| Variable | | Obs Mean Std. dev. Min Max |
| | | |
| weight | | 74 3019.459 777.1936 1760 4840 |
.
local meanwei=r(mean)
.
summarize length
| Variable | | Obs Mean Std. dev. Min Max |
| | | |
| length | | 74 187.9324 22.26634 142 233 |
.
local meanlen=r(mean)
.
local f=`meanwei'*_b[weight]+`meanlen'*_b[length]+_b[_cons]
.
display "weight: eyex = " (`meanwei'*_b[weight])/`f'
weight: eyex = -.54604966
.
display "length: eyex = " (`meanlen'*_b[length])/`f'
length: eyex = -.70235175
.
margins, eyex(weight length) atmeans nose
Conditional marginal effects Number of obs = 74
Expression: Linear prediction, predict()
ey/ex wrt: weight length
At: weight = 3019.459 (mean)
length = 187.9324 (mean)
|
| | | ey/ex |
| | | |
| weight | | -.5460497 |
| length | | -.7023518 |
|
We can plot the elasticities as functions of the independent variables
using margins with the at() option followed by marginsplot.
In the following example, there are two independent variables, and
we plot the elasticity of each independent variable at the mean of the other
variable.
. sysuse auto, clear
(1978 automobile data)
. regress mpg weight length
| Source | | SS df MS | Number of obs = 74 |
| | | | F(2, 71) = 69.34 |
| Model | | 1616.08062 2 808.040312 | Prob > F = 0.0000 |
| Residual | | 827.378835 71 11.653223 | R-squared = 0.6614 |
| | | | Adj R-squared = 0.6519 |
| Total | | 2443.45946 73 33.4720474 | Root MSE = 3.4137 |
|
| mpg | | Coefficient Std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | -.0038515 .001586 -2.43 0.018 -.0070138 -.0006891 |
| length | | -.0795935 .0553577 -1.44 0.155 -.1899736 .0307867 |
| _cons | | 47.88487 6.08787 7.87 0.000 35.746 60.02374 |
|
.
margins, eyex(weight) at(weight = (1750(250)5000) (mean) length) noatlegend
Conditional marginal effects Number of obs = 74
Model VCE : OLS
Expression : Linear prediction, predict()
ey/ex w.r.t. : weight
|
| | | Delta-method |
| | | ey/ex std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | |
| _at | | |
| 1 | | -.2573869 .0862872 -2.98 0.004 -.4294388 -.085335 |
| 2 | | -.3053854 .1062865 -2.87 0.005 -.5173147 -.0934561 |
| 3 | | -.3571938 .1292519 -2.76 0.007 -.6149147 -.0994728 |
| 4 | | -.4132845 .1557291 -2.65 0.010 -.7237996 -.1027694 |
| 5 | | -.4742113 .1863901 -2.54 0.013 -.8458627 -.10256 |
| 6 | | -.540628 .2220688 -2.43 0.017 -.9834206 -.0978354 |
| 7 | | -.6133115 .2638095 -2.32 0.023 -1.139333 -.0872902 |
| 8 | | -.6931926 .312933 -2.22 0.030 -1.317163 -.0692218 |
| 9 | | -.7813962 .3711274 -2.11 0.039 -1.521403 -.0413892 |
| 10 | | -.8792944 .4405755 -2.00 0.050 -1.757777 -.0008119 |
| 11 | | -.9885785 .5241373 -1.89 0.063 -2.033679 .0565216 |
| 12 | | -1.111358 .6256144 -1.78 0.080 -2.358797 .1360822 |
| 13 | | -1.250295 .7501418 -1.67 0.100 -2.746036 .2454448 |
| 14 | | -1.408807 .9047836 -1.56 0.124 -3.212894 .3952802 |
|
.
marginsplot, noci xlabel(2000(1000)5000) saving(weight, replace) nodraw
Variables that uniquely identify margins: weight
(file weight.gph not found)
file weight.gph saved
.
margins, eyex(length) at(length = (140(10)240) (mean) weight) noatlegend
Conditional marginal effects Number of obs = 74
Model VCE: OLS
Expression: Linear prediction, predict()
ey/ex wrt: length
|
| | | Delta-method |
| | | ey/ex std. err. t P>|t| [95% conf. interval] |
| | | |
| length | | |
| _at | | |
| 1 | | -.4437283 .2618243 -1.69 0.095 -.9657911 .0783345 |
| 2 | | -.4909849 .2991911 -1.64 0.105 -1.087555 .1055853 |
| 3 | | -.5414398 .3411019 -1.59 0.117 -1.221578 .138698 |
| 4 | | -.5954291 .3882531 -1.53 0.130 -1.369584 .1787255 |
| 5 | | -.6533377 .4414755 -1.48 0.143 -1.533615 .2269394 |
| 6 | | -.7156083 .5017656 -1.43 0.158 -1.7161 .2848838 |
| 7 | | -.7827532 .5703263 -1.37 0.174 -1.919952 .3544452 |
| 8 | | -.855368 .64862 -1.32 0.191 -2.14868 .4379437 |
| 9 | | -.9341494 .7384374 -1.27 0.210 -2.406552 .5382529 |
| 10 | | -1.019918 .8419885 -1.21 0.230 -2.698795 .6589597 |
| 11 | | -1.113646 .9620256 -1.16 0.251 -3.03187 .8045788 |
|
.
marginsplot, noci xlabel(140(20)240) saving(length, replace) nodraw
Variables that uniquely identify margins: length
(file length.gph not found)
file length.gph saved
.
graph combine weight.gph length.gph, ycommon
How do the elasticities computed by margins relate to the coefficients of
the loglinear model?
The term elasticity has also been used to describe the coefficient of the
model
ln(y) = b0 + b1*ln(x)
This is called a constant elasticity model. When we do
y = c0 + c1*x
and compute d(ln(f))/d(ln(x)), where f is the linear predictor, this is a
function of x. We can evaluate this function at any value of x we please.
This is a varying elasticity model.
In the following example, we compute the variable elasticity using
margins, but rather than just computing it at just one point, the mean of
the independent variable, we compute it at many values of
the independent variable. We also plot it so we can get a good feel for the
elasticity as a function of the independent variable.
Also, we fit the loglinear model and plot the coefficient on the
graph. The final piece we add to the graph is to mark the mean of the
independent variable and the value of the varying elasticity there.
. sysuse auto, clear
(1978 automobile data)
. keep mpg weight
. sum weight
| Variable | | Obs Mean Std. dev. Min Max |
| | | | |
| weight | | 74 3019.459 777.1936 1760 4840 |
.
local mean=r(mean)
.
* ---constant elasticity model --------------------
.
gen lnmpg=ln(mpg)
.
gen lnwei=ln(weight)
.
regress lnmpg lnwei
| Source | | SS df MS | Number of obs = 74 |
| | | | F(1, 72) = 179.41 |
| Model | | 3.52612925 1 3.52612925 | Prob > F = 0.0000 |
| Residual | | 1.4150941 72 .019654085 | R-squared = 0.7136 |
| | | | Adj R-squared = 0.7096 |
| Total | | 4.94122335 73 .067687991 | Root MSE = .14019 |
|
| lnmpg | | Coefficient Std. err. t P>|t| [95% conf. interval] |
| | | |
| lnwei | | -.8251737 .061606 -13.39 0.000 -.9479829 -.7023645 |
| _cons | | 9.608391 .4918087 19.54 0.000 8.627989 10.58879 |
|
.
gen marg_cons=_b[lnwei]
.
* ---varying elasticity model----------------------
.
regress mpg weight
| Source | | SS df MS | Number of obs = 74 |
| | | | F(1, 72) = 134.62 |
| Model | | 1591.9902 1 1591.9902 | Prob > F = 0.0000 |
| Residual | | 851.469256 72 11.8259619 | R-squared = 0.6515 |
| | | | Adj R-squared = 0.6467 |
| Total | | 2443.45946 73 33.4720474 | Root MSE = 3.4389 |
|
| mpg | | Coefficient Std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | -.0060087 .0005179 -11.60 0.000 -.0070411 -.0049763 |
| _cons | | 39.44028 1.614003 24.44 0.000 36.22283 42.65774 |
|
.
* ----elasticity at the mean-----------------------
.
margins, eyex(weight) atmeans
Conditional marginal effects Number of obs = 74
Model VCE : OLS
Expression : Linear prediction, predict()
ey/ex w.r.t. : weight
at : weight = 3019.459 (mean)
|
| | | Delta-method |
| | | ey/ex std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | -.8518915 .0751441 -11.34 0.000 -1.001689 -.7020944 |
|
.
matrix A=r(b)
.
gen marg_mean = A[1,1]
.
* ----elasticity at different values of weight ----
.
margins, eyex(weight) at(weight = (1750(250)5000)) noatlegend
Conditional marginal effects Number of obs = 74
Model VCE : OLS
Expression : Linear prediction, predict()
ey/ex w.r.t. : weight
|
| | | Delta-method |
| | | ey/ex std. err. t P>|t| [95% conf. interval] |
| | | |
| weight | | |
| _at | | |
| 1 | | -.3635323 .0236104 -15.40 0.000 -.4105988 -.3164658 |
| 2 | | -.4382239 .0300205 -14.60 0.000 -.4980686 -.3783791 |
| 3 | | -.5215725 .0378009 -13.80 0.000 -.5969273 -.4462178 |
| 4 | | -.615176 .0473276 -13.00 0.000 -.7095219 -.5208302 |
| 5 | | -.721051 .0591092 -12.20 0.000 -.8388829 -.603219 |
| 6 | | -.8417798 .0738467 -11.40 0.000 -.9889906 -.694569 |
| 7 | | -.9807243 .0925265 -10.60 0.000 -1.165172 -.7962761 |
| 8 | | -1.142343 .1165684 -9.80 0.000 -1.374718 -.9099685 |
| 9 | | -1.332681 .1480732 -9.00 0.000 -1.627859 -1.037502 |
| 10 | | -1.560137 .1902484 -8.20 0.000 -1.93939 -1.180884 |
| 11 | | -1.836744 .2481783 -7.40 0.000 -2.331478 -1.342009 |
| 12 | | -2.180362 .3302937 -6.60 0.000 -2.838791 -1.521934 |
| 13 | | -2.6187 .4513707 -5.80 0.000 -3.518491 -1.718909 |
| 14 | | -3.197182 .6391756 -5.00 0.000 -4.471355 -1.923008 |
|
.
marginsplot, noci xlabel(2000(1000)5000) addplot(line marg_cons wei /*
> */ || line marg_mean wei, xline(`mean') /*
> */ legend(order(1 "varying" 2 "constant" 3 "mean") rows(3)))
Variables that uniquely identify margins: weight
