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 The QUANTREG Procedure

## Example 73.1 Comparison of Algorithms

This example illustrates and compares the three algorithms for regression estimation available in the QUANTREG procedure. The simplex algorithm is the default because of its stability. Although this algorithm is slower than the interior point and smoothing algorithms for large data sets, the difference is not as significant for data sets with fewer than 5,000 observations and 50 variables. The simplex algorithm can also compute the entire quantile process, which is shown in Example 73.2.

The following statements generate 1,000 random observations. The first 950 observations are from a linear model, and the last 50 observations are significantly biased in the -direction. In other words, 5 of the observations are contaminated with outliers.

```   data a (drop=i);
do i=1 to 1000;
x1=rannor(1234);
x2=rannor(1234);
e=rannor(1234);
if i > 950 then y=100 + 10*e;
else y=10 + 5*x1 + 3*x2 + 0.5 * e;
output;
end;
run;
```

The following statements invoke the QUANTREG procedure to fit a median regression model with the default simplex algorithm. They produce the results in Output 73.1.1 through Output 73.1.3.

```proc quantreg data=a;
model y = x1 x2;
run;
```

Output 73.1.1 displays model information and summary statistics for variables in the model. It indicates that the simplex algorithm is used to compute the optimal solution and the rank method is used to compute confidence intervals of the parameters.

By default, the QUANTREG procedure fits a median regression model. This is indicated by the quantile value 0.5 in Output 73.1.2, which also displays the objective function value and the predicted value of the response at the means of the covariates.

Output 73.1.3 displays parameter estimates and confidence limits. These estimates are reasonable, which indicates that median regression is robust to the 50 outliers.

Output 73.1.1 Model Fit Information and Summary Statistics with Simplex Algorithm
The QUANTREG Procedure

Model Information
Data Set WORK.A
Dependent Variable y
Number of Independent Variables 2
Number of Observations 1000
Optimization Algorithm Simplex
Method for Confidence Limits Inv_Rank

Summary Statistics
Variable Q1 Median Q3 Mean Standard
Deviation
MAD
x1 -0.6546 0.0230 0.7099 0.0222 0.9933 1.0085
x2 -0.7891 -0.0747 0.6839 -0.0401 1.0394 1.0857
y 6.1045 10.6936 14.9569 14.4864 20.4087 6.5696

Output 73.1.2 Quantile and Objective Function with Simplex Algorithm
Quantile and Objective Function
Quantile 0.5
Objective Function 2441.1927
Predicted Value at Mean 10.0259

Output 73.1.3 Parameter Estimates with Simplex Algorithm
Parameter Estimates
Parameter DF Estimate 95% Confidence Limits
Intercept 1 10.0364 9.9959 10.0756
x1 1 5.0106 4.9602 5.0388
x2 1 3.0294 2.9944 3.0630

The following statements refit the model by using the interior point algorithm:

```proc quantreg algorithm=interior(tolerance=1e-6)
ci=none data=a;
model y = x1 x2 / itprint nosummary;
run;
```

The TOLERANCE= option specifies the stopping criterion for convergence of the interior point algorithm, which is controlled by the duality gap. Although the default criterion is 1E8, the value 1E6 is often sufficient. The ITPRINT option requests the iteration history for the algorithm. The option CI=NONE suppresses the computation of confidence limits, and the option NOSUMMARY suppresses the table of summary statistics.

Output 73.1.4 displays model fit information.

Output 73.1.4 Model Fit Information with Interior Point Algorithm
The QUANTREG Procedure

Model Information
Data Set WORK.A
Dependent Variable y
Number of Independent Variables 2
Number of Observations 1000
Optimization Algorithm Interior

Output 73.1.5 displays the iteration history of the interior point algorithm. Note that the duality gap is less than 1E6 in the final iteration. The table also provides the number of iterations, the number of corrections, the primal step length, the dual step length, and the objective function value at each iteration.

Output 73.1.5 Iteration History for the Interior Point Algorithm
Iteration History of Interior Point Algorithm
Duality
Gap
Iter Correction Primal
Step
Dual
Step
Objective
Function
2623 1 1 0.3113 0.4910 3303.4688
3215 2 2 0.0427 1.0000 2461.3774
1127 3 3 0.9882 0.3653 2451.1337
760.88658 4 4 0.3381 1.0000 2442.8104
77.10290 5 5 1.0000 0.8916 2441.2627
8.43666 6 6 0.9370 0.8381 2441.2085
1.82868 7 7 0.8375 0.7674 2441.1985
0.40584 8 8 0.6980 0.8636 2441.1948
0.09550 9 9 0.9438 0.5955 2441.1930
0.00665 10 10 0.9818 0.9304 2441.1927
0.0002248 11 11 0.9179 0.9994 2441.1927
5.44651E-8 12 12 1.0000 1.0000 2441.1927

Output 73.1.6 displays the parameter estimates obtained with the interior point algorithm, which are identical to those obtained with the simplex algorithm.

Output 73.1.6 Parameter Estimates with Interior Point Algorithm
Parameter Estimates
Parameter DF Estimate
Intercept 1 10.0364
x1 1 5.0106
x2 1 3.0294

The following statements refit the model by using the smoothing algorithm. They produce the results in Output 73.1.7 through Output 73.1.9.

```proc quantreg algorithm=smooth(rratio=.5) ci=none data=a;
model y = x1 x2 / itprint nosummary;
run;
```

The RRATIO= option controls the reduction speed of the threshold. Output 73.1.7 displays the model fit information.

Output 73.1.7 Model Fit Information with Smoothing Algorithm
The QUANTREG Procedure

Model Information
Data Set WORK.A
Dependent Variable y
Number of Independent Variables 2
Number of Observations 1000
Optimization Algorithm Smooth

Output 73.1.8 displays the iteration history of the smoothing algorithm. The threshold controls the convergence. Note that the thresholds decrease by a factor of at least 0.5, the value specified with the RRATIO= option. The table also provides the number of iterations, the number of factorizations, the number of full updates, the number of partial updates, and the objective function value in each iteration. For details concerning the smoothing algorithm, refer to Chen (2007).

Output 73.1.8 Iteration History for the Smoothing Algorithm
Iteration History of Smoothing Algorithm
Threshold Iter Refac Full
Update
Partial
Update
Objective
Function
227.24557 1 1 1000 0 4267.0988
116.94090 15 4 1480 2420 3631.9653
1.44064 17 4 1480 2583 2441.4719
0.72032 20 5 1980 2598 2441.3315
0.36016 22 6 2248 2607 2441.2369
0.18008 24 7 2376 2608 2441.2056
0.09004 26 8 2446 2613 2441.1997
0.04502 28 9 2481 2617 2441.1971
0.02251 30 10 2497 2618 2441.1956
0.01126 32 11 2505 2620 2441.1946
0.00563 34 12 2510 2621 2441.1933
0.00281 35 13 2514 2621 2441.1930
0.0000846 36 14 2517 2621 2441.1927
1E-12 37 14 2517 2621 2441.1927

Output 73.1.9 displays the parameter estimates obtained with the smoothing algorithm, which are identical to those obtained with the simplex and interior point algorithms. All three algorithms should have the same parameter estimates unless the problem does not have a unique solution.

Output 73.1.9 Parameter Estimates with Smoothing Algorithm
Parameter Estimates
Parameter DF Estimate
Intercept 1 10.0364
x1 1 5.0106
x2 1 3.0294

The interior point algorithm and the smoothing algorithm offer better performance than the simplex algorithm for large data sets. Refer to Chen (2004) for more details on choosing an appropriate algorithm on the basis of data set size. All three algorithms should have the same parameter estimates, unless the optimization problem has multiple solutions.

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