Title | Fitting a regression with interval constraints | |

Author | Isabel Canette, StataCorp |

**Note:**
The examples in this tutorial are for illustration purposes only. If you need to fit a linear regression with interval constraints, use the Stata command
nl
as explained in
How do I fit a linear regression with interval (inequality) constraints in Stata?

If you are interested in fitting a linear regression with a linear
constraint, you can use the command
cnsreg. However,
sometimes you may need to fit a model for which one of the
coefficients should be positive or between zero and one. You can fit such a model
by reparameterizing the coefficients. This tutorial
explains the steps to fit this model by using the Stata command
ml. For simplicity, I explain how
to set interval constraints on a linear regression. However, this method could
be applied to other kinds of regression (e.g., probit, logit, Poisson, tobit),
provided that they can be fitted by maximum likelihood. As stated above,
if you want to set nonlinear constraints on a linear regression, you can use
nl, which would not require any programming.
Starting with Stata 13, the **mlexp** command allows you to fit maximum
likelihood models by evaluating an expression. At the end of each section,
I include the syntax to fit the models using **mlexp**.

This FAQ is organized as follows:

- Introduction
- Prerequisites
- Our introductory example
- Application: Setting a model where parameters are proportions
- Final remarks

I will start by presenting an example on how to use ml to fit a linear regression. The code in this example can be modified to set interval constraints. I will explain how to perform this modification by showing the steps to restrict one coefficient to be positive.

Finally, I will present an example where the coefficients are restricted to be proportions; that is, they are constrained to be positive numbers that add up to one.

If you know (from the nature of the problem) that the constraints you want to set are true, the following method will allow you to set these restrictions in the model. However, if the data do not support the validity of these constraints, the algorithm for the restricted model probably won’t achieve convergence.

There are essentially two situations where we may want to set interval constraints. One case is when the numerical global maximum of the likelihood actually is where we want it to be, but we want to help the algorithm avoid difficult spots somewhere else. This case is similar to the situation of estimating the standard deviation in a linear regression, where the maximum is positive, but we set the restriction to help the algorithm achieve convergence. The other situation is when we know that the true parameter is in a certain region, but for numerical reasons the global maximum of the likelihood may be somewhere else. We are interested not in finding a global maximum but in finding a maximum in a certain subset. This is the case of the example of proportions, considered in section 4.

If you also need to set linear constraints, you can use the option constraint of the command ml.

This tutorial assumes that you are familiar with the command
ml. I will review some characteristics of the
estimation by using ml here. However, if you have
never used the Stata command ml, see [R] **ml**.

Technical notes explain some details on the output; you can skip them in a first reading.

Let’s assume that we want to reproduce the following estimation,

. sysuse auto, clear . regress price mpg displacement

but we want to make sure that the coefficient of variable mpg will be negative.

Because we are going to use method d0, we will borrow the program mynormal_d0.ado from the book Maximum Likelihood Estimation with Stata, Fourth Edition, by Gould, Pitblado, and Poi (you can download this file from the book’s website). Although the syntax for method lf is simpler, if we adapt an lf program to fit a model with interval constraints, the good properties of lf will be lost (see technical note below); some preliminary simulations showed that, when adapted to our problem, method d0 is better for convergence.

**Technical note:** Whenever the likelihood meets the linear form,
method lf is efficient. With lf, the
derivatives are computed at the equation level, and the chain rule is applied to
obtain the derivatives with respect to the parameters. This method produces
high-quality derivatives and therefore accurate estimators. When we
modify a maximum likelihood program to meet interval restrictions according
to the procedure explained below, the equations are disassembled; then, even
when using an lf program, we will force
ml to compute numerical derivatives directly with
respect to the coefficients, losing the numerical benefits of
lf.

Let’s start by fitting the model without restrictions, using regress.

. sysuse auto, clear(1978 Automobile Data). regress price mpg displacement

Source | SS df MS | Number of obs = 74 | |

F( 2, 71) = 13.35 | |||

Model | 173587098 2 86793549.2 | Prob > F = 0.0000 | |

Residual | 461478298 71 6499694.33 | R-squared = 0.2733 | |

Adj R-squared = 0.2529 | |||

Total | 635065396 73 8699525.97 | Root MSE = 2549.4 |

price | Coef. Std. Err. t P>|t| [95% Conf. Interval] | |

mpg | -121.1833 72.78844 -1.66 0.100 -266.3193 23.95276 | |

displacement | 10.50885 4.58548 2.29 0.025 1.365658 19.65203 | |

_cons | 6672.766 2299.72 2.90 0.005 2087.254 11258.28 | |

Now let’s fit the same model by using ml:

/* begin do file */ cscript program mynormal_d0 version 16 args todo b lnf tempname lnsigma sigma tempvar mu mleval `mu' = `b', eq(1) mleval `lnsigma' = `b', eq(2) scalar quietly { scalar `sigma' = exp(`lnsigma') mlsum `lnf' = ln( normalden($ML_y1,`mu',`sigma') ) } end sysuse auto ml model d0 mynormal_d0 (xb: price = mpg displacement) (lnsigma:), /// diparm(lnsigma, exp label(sigma)) ml maximize /* end do file */

Here are the results:

Number of obs = 74 Wald chi2(2) = 27.84 Log likelihood = -683.89903 Prob > chi2 = 0.0000

price | Coef. Std. Err. z P>|z| [95% Conf. Interval] | |

xb | ||

mpg | -121.1831 71.29769 -1.70 0.089 -260.924 18.55784 | |

displacement | 10.50884 4.491567 2.34 0.019 1.705527 19.31215 | |

_cons | 6672.764 2252.621 2.96 0.003 2257.709 11087.82 | |

lnsigma | ||

_cons | 7.82294 .0821995 95.17 0.000 7.661832 7.984048 | |

sigma | 2497.236 205.2714 2125.648 2933.782 |

** Technical note:** You will see some differences between the output of
ml and the output of
regress. First, the standard errors
are different because a different number of degrees of freedom was used to
compute the variance of the errors. The variance for the errors in
regress and ml is estimated,
respectively, as v1 and
v2, where

v1 = e'e/(N-k) v2 = e'e/(N)

Here e is the vector of residuals, N is the number of observations, and k is the number of regressors, including the constant.

Second, the confidence intervals reported by
regress are based on the *t* distribution, and
ml uses the Gaussian distribution to compute them.

Third, regress performs an *F* test, and
ml performs a
Wald test. See the following FAQ to learn how these tests relate to each
other: ** Why
does test sometimes produce chi-squared and other times F statistics?**

Let’s have a closer look at the code for ml; the line

ml model d0 mynormal_d0 (xb: price = mpg displacement) (lnsigma:)

introduces two equations. The first equation,

(xb: price = mpg displacement)

indicates the following:

- The name of the equation is
**xb**; this name is displayed in the output. - The dependent variable is
**price**; this variable is referred to as**$ML_y1**in the program**mynormal_d0**. - This equation has two independent variables:
**mpg**and**displacement**.

Therefore, when the line

mleval `mu' = `b', eq(1)

is run in the program, the local variable**`mu'**is evaluated as a linear combination of these two variables. Because of the second line in**mynormal_d0**,

args todo b lnf

the local macro**`b'**will contain the matrix of parameters. Therefore, what the evaluation

mleval `mu' = `b', eq(1)

does, essentially, is

replace `mu' = `b'[1,1]*mpg + `b'[1,2]*displacement + `b'[1,3]

The second equation,

(lnsigma:)

indicates that

- the name of this equation is
**lnsigma**, and - there are not independent variables in this equation.

Because this is a constant-only equation, there is no need to generate a whole variable for it in the evaluation. Evaluating it as scalar is more efficient, as in

mleval `lnsigma' = `b', eq(2) scalar

This is essentially the same as

scalar `lnsigma' = `b'[1,4]

This parameter is used in the line

scalar double `sigma' = exp(`lnsigma')

We are not estimating the standard deviation **(sigma)**, but its
logarithm **(lnsigma)**. This trick allows us to express the standard
deviation as an exponent **(exp(`lnsigma'))** and guarantees that
**sigma** will be positive. We make sure that the algorithm will not
search for **sigma **in the negative subspace. We want to use this trick
to apply an inequality constraint to a coefficient.
The model above could also be fit by using the **mlexp** syntax:

mlexp (ln(normalden(price, {xb:mpg displacement}+{b0}, exp({lnsigma}))))

In this step, we set up a separate equation for the coefficient that we will restrict.

In the program **mynormal_d0**, the coefficient of **mpg** is included
in equation 1. To set a restriction on it, we need to extract
this coefficient from the equation. More precisely, we need to include this
coefficient as a scalar parameter in the model, that is

ml model d0 mynormal_b (a: ) (xb: price = displacement ) (lnsigma:)

where the constant-only equation **(a: )** corresponds to the coefficient
of **mpg**. The question of how to include the variable itself remains;
as we cannot introduce this variable in any of the equations, we will need
to use a global macro. Look at the following code:

/* begin do file */ cscript program mynormal_b version 16 args todo b lnf tempname a lnsigma sigma tempvar xb mu mleval `a' = `b', eq(1) scalar mleval `xb' = `b', eq(2) mleval `lnsigma' = `b', eq(3) scalar quietly { generate double `mu' = `xb' +`a'*$x2 scalar `sigma' = exp(`lnsigma') mlsum `lnf' = ln( normalden($ML_y1,`mu',`sigma') ) } end sysuse auto global x2 mpg ml model d0 mynormal_b (a: ) (xb: price = displacement ) (lnsigma:), /// diparm(lnsigma, exp label(sigma)) ml maximize /* end do file */

Here are the estimation results:

Number of obs = 74 Wald chi2(0) = . Log likelihood = -683.89903 Prob > chi2 = .

price | Coef. Std. Err. z P>|z| [95% Conf. Interval] | |

a | ||

_cons | -121.1833 71.29771 -1.70 0.089 -260.9242 18.55769 | |

xb | ||

displacement | 10.50885 4.491569 2.34 0.019 1.705532 19.31216 | |

_cons | 6672.766 2252.621 2.96 0.003 2257.709 11087.82 | |

lnsigma | ||

_cons | 7.82294 .0821995 95.17 0.000 7.661832 7.984048 | |

sigma | 2497.237 205.2716 2125.649 2933.784 | |

**Technical note:** When we use **mynormal_b**,
missing values are reported for the Wald chi2 test. ml,
by default, reports this statistic for all the coefficients except the
intercept in the first equation; the number of degrees of freedom is
reported as zero because the first equation has only the intercept.

To perform a Wald test on the coefficients, we can use the command
**test**
after the estimation:

. matrix list e(b)e(b)[1,4] a: xb: xb: lnsigma: _cons displacement _cons _cons y1 -121.18326 10.508845 6672.7659 7.8229403. test ([a]_cons) ([xb]displacement)( 1) [a]_cons = 0 ( 2) [xb]displacement = 0 chi2( 2) = 27.84 Prob > chi2 = 0.0000

These are the values reported by ml when running
**mynormal_d0**.

The model above could also be fit by using the following **mlexp**
syntax:

mlexp (ln(normalden(price, {a}*mpg+{b1}*displacement+{b0}, exp({lnsigma}))))

Now we only have to change one line in the program: we substitute
**`a'** by **-exp(`a')** in the evaluation and display the
transformed parameter after the estimation:

/* begin do file */ cscript program mynormal_c version 16 args todo b lnf tempname a lnsigma sigma tempvar xb mu mleval `a' = `b', eq(1) scalar mleval `xb' = `b', eq(2) mleval `lnsigma' = `b', eq(3) quietly { generate double `mu' = `xb' -exp(`a')*$x2 scalar `sigma' = exp(`lnsigma') mlsum `lnf' = ln( normalden($ML_y1,`mu',`sigma') ) } end sysuse auto global x2 mpg ml model d0 mynormal_c (a: ) (xb: price = displacement ) (lnsigma:), /// diparm(lnsigma, exp label(sigma)) ml init lnsigma:_cons=7.8 ml maximize /* end do file */

Here we used the global macro **x2** to incorporate the variable
**mpg**; when including a variable manually (i.e., not through the
standard syntax of ml), you must check for missing values.

Here are the results:

Number of obs = 74 Wald chi2(0) = . Log likelihood = -683.89903 Prob > chi2 = .

price | Coef. Std. Err. z P>|z| [95% Conf. Interval] | |

a | ||

_cons | 4.797299 .58835 8.15 0.000 3.644154 5.950444 | |

xb | ||

displacement | 10.50887 4.491572 2.34 0.019 1.705552 19.31219 | |

_cons | 6672.748 2252.625 2.96 0.003 2257.685 11087.81 | |

lnsigma | ||

_cons | 7.82294 .0821995 95.17 0.000 7.661832 7.984048 | |

sigma | 2497.237 205.2717 2125.649 2933.784 | |

Now we can use the command nlcom to display the estimate for the coefficient:

. nlcom -exp([a]_cons)_nl_1: -exp([a]_cons)

price | Coef. Std. Err. z P>|z| [95% Conf. Interval] | |

_nl_1 | -121.1827 71.29782 -1.70 0.089 -260.9238 18.55855 | |

We may want to help ml a little by providing initial values. For
example, if we believe that the variance reported by regress is
reasonable, we can include it as an initial value with **ml init**. Here
we used the residual MS reported by regress to set the logarithm of
the variance, that is

. display log(sqrt(6499694.33))7.8436329

The model above could also be fit by using the following **mlexp**
syntax:

mlexp (ln(normalden(price, -exp({a})*mpg + {b1}*displacement +{b0}, exp({lnsigma}))))

Now we will apply the previous ideas to a particular example. Let’s assume that we want to fit the model

y = a1*x1 + a2*x2 + a3*x3 + a4

where a1, a2, and a3 are proportions, i.e., values between zero and one that add up to one. Given this restriction, we need to estimate only two parameters (because a1 = 1 − (a2 + a3)). We need to find a proper parameterization for our restriction. A nice choice is the parameterization used by the command mlogit that fits multinomial logit models.

In this approach, instead of estimating a2 and a3, we estimate
**t2** and **t3**, where:

a2 = exp(t2)/(1+exp(t2)+exp(t3)) a3 = exp(t3)/(1+exp(t2)+exp(t3)) a1 = 1/(1+exp(t2)+exp(t3))

Verifying that this parameterization guarantees our restriction is straightforward.

In the following code, the function mynormal_c can be used with the command ml to perform this estimation; the wrapper regprop runs this program and displays the values of interest.

/* begin do file */ cscript set seed 12345 set obs 1000 generate x1 = invnormal(uniform()) generate x2 = invnormal(uniform()) generate x3 = invnormal(uniform()) generate ep = invnormal(uniform()) generate y = .2*x1 + .5*x2 + .3*x3 + 1 + ep regress y x1 x2 x3 program mynormal_c version 16 args todo b lnf tempname t2 t3 a4 lnsigma sigma den a1 a2 a3 tempvar mu mleval `t2' = `b', eq(1) scalar mleval `t3' = `b', eq(2) scalar mleval `a4' = `b', eq(3) scalar mleval `lnsigma' = `b', eq(4) scalar `den' = 1+ exp(`t2') + exp(`t3') scalar `a1' = 1/`den' scalar `a2' = exp(`t2')/`den' scalar `a3' = exp(`t3')/`den' generate double `mu' = `a1'*$x_001 + `a2'*$x_002 + `a3'*$x_003 + `a4' scalar `sigma' = exp(`lnsigma') mlsum `lnf' = ln( normalden($ML_y1,`mu',`sigma') ) end program regprop version 16 syntax varlist gettoken lhs rhs: varlist global x_001 : word 1 of `rhs' global x_002 : word 2 of `rhs' global x_003 : word 3 of `rhs' local f1 func(1/(1+exp(@1)+exp(@2))) local der1 der( -exp(@1)/(1+exp(@1)+exp(@2))^2 /// -exp(@2)/(1+exp(@1)+exp(@2))^2 ) local f2 func(exp(@1)/(1+exp(@1)+exp(@2))) local der2 der( exp(@1)*(1+exp(@2))/(1+exp(@1)+exp(@2))^2 /// -exp(@1)*exp(@2)/(1+exp(@1)+exp(@2))^2 ) local f3 func(exp(@2)/(1+exp(@1)+exp(@2))) local der3 der( -exp(@1)*exp(@2)/(1+exp(@1)+exp(@2))^2 /// exp(@2)*(1+exp(@1))/(1+exp(@1)+exp(@2))^2 ) ml model d0 mynormal_c (t2:) (t3:) (c: `lhs' = ) (lnsigma:), /// diparm( t2 t3, `f1' `der1' label ($x_001) ci(logit) ) /// diparm( t2 t3, `f2' `der2' label ($x_002) ci(logit) ) /// diparm( t2 t3, `f3' `der3' label ($x_003) ci(logit) ) ml maximize end regprop y x1 x2 x3 /* end do file */

Here are the results:

Number of obs = 1000 Wald chi2(0) = . Log likelihood = -1414.503 Prob > chi2 = .

y | Coef. Std. Err. z P>|z| [95% Conf. Interval] | |

t2 | ||

_cons | .7524279 .1504271 5.00 0.000 .4575961 1.04726 | |

t3 | ||

_cons | .2617819 .1745923 1.50 0.134 -.0804128 .6039765 | |

c | ||

_cons | .9913625 .0314883 31.48 0.000 .9296466 1.053078 | |

lnsigma | ||

_cons | -.0044355 .0223607 -0.20 0.843 -.0482616 .0393906 | |

x1 | .2261733 .0259554 .1793569 .2810251 | |

x2 | .4799727 .0262131 .4289861 .5313799 | |

x3 | .2938541 .0256475 .2461986 .3464938 | |

Here the option diparm was used to display the actual estimates of the coefficients; besides aesthetic issues, in this case, this option is preferred to the command nlcom because of its flexibility. For example, the option ci(logit) allows us to make sure that the limits of the confidence intervals will also be between zero and one. The syntax for the option diparm is similar to the syntax for the command _diparm.

However, the confidence intervals reported by the option diparm are asymptotically equivalent to confidence intervals reported by nlcom; therefore, you can also use nlcom as in the example in section 1.3.

The model above could also be fit by using the following syntax:

local den (1+ exp({t2}) + exp({t3})) local a1 1/`den' local a2 exp({t2})/`den' local a3 exp({t3})/`den' mlexp (ln(normalden(y, `a1'*x1 + `a2'*x2 + `a3'*x3 + {c}, exp({lnsigma}) )))

As you probably noticed, the tricky part of this method is finding an appropriate parameterization for the coefficients in your restriction. You may always want to take into account the exponential function to set inequality restrictions, the inverse logit transformation and the hyperbolic tangent transformation to set interval restrictions, and the multinomial logit transformation to set simultaneous interval restrictions, as in the previous section. For example, if you need to set the restrictions:

a>3 3<b<10

you can use the transformations:

a = exp(a1) + 3 b = 3 + 7*invlogit(b1)

Also, for simplicity, I wrote all the examples by using method
d0; however, to achieve convergence, you
may need to use methods **d1** or d2, in
addition to setting good initial values.