This page covers Example 2 from the interval regression data analysis example.
The issue addressed here is how to analyze data that has both interval-censored and left-censored and right-censored values.
Let’s look at the data and then perform an interval regression with robust standard errors.
use https://stats.idre.ucla.edu/stat/stata/dae/intregex2, clear list lgpa ugpa, clean lgpa ugpa 1. 2.5 3 2. 3.4 3.8 3. 2.5 3 4. . 2 5. 3 3.4 6. 3.4 3.8 7. 3.8 4 8. 2 2.5 9. 3 3.4 10. 3.4 3.8 11. 2 2.5 12. 2 2.5 13. 2 2.5 14. 2.5 3 15. 2.5 3 16. 2.5 3 17. 3.4 3.8 18. 2.5 3 19. 2 2.5 20. 3 3.4 21. 3.4 3.8 22. 4 . 23. 2 2.5 24. 3 3.4 25. 3.4 3.8 26. 2 2.5 27. 2 2.5 28. 2 2.5 29. 2.5 3 30. 2.5 3
Note observations 4 and 22 which are left-censored and right-censored respectively. You indicate censoring by setting either the lower or upper interval value to missing.
intreg lgpa ugpa write rating read, robust Fitting constant-only model: Iteration 0: log pseudolikelihood = -51.784888 Iteration 1: log pseudolikelihood = -51.75397 Iteration 2: log pseudolikelihood = -51.753956 Iteration 3: log pseudolikelihood = -51.753956 Fitting full model: Iteration 0: log pseudolikelihood = -36.722734 Iteration 1: log pseudolikelihood = -36.427115 Iteration 2: log pseudolikelihood = -36.425896 Iteration 3: log pseudolikelihood = -36.425896 Interval regression Number of obs = 30 Wald chi2(3) = 48.39 Log pseudolikelihood = -36.425896 Prob > chi2 = 0.0000 ------------------------------------------------------------------------------ | Robust | Coef. Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------------- write | .0054281 .001462 3.71 0.000 .0025627 .0082936 rating | .0185163 .0109571 1.69 0.091 -.0029592 .0399917 read | .0024199 .0011216 2.16 0.031 .0002216 .0046182 _cons | .7927504 .5138823 1.54 0.123 -.2144405 1.799941 -------------+---------------------------------------------------------------- /lnsigma | -1.053316 .1300213 -8.10 0.000 -1.308153 -.7984792 -------------+---------------------------------------------------------------- sigma | .3487792 .0453487 .2703188 .4500128 ------------------------------------------------------------------------------ Observation summary: 1 left-censored observation 0 uncensored observations 1 right-censored observation 28 interval observations
Note the observation summary at the bottom of the output that indicates that there was one left-censored, one right-censored, and 28 interval-censored values.
You should compare these results with the results from Example 3 that contained only interval-censored data. Because of the similarities in the output between this example and Example 3, the interpretation and write-up of the analysis will be the same as for Example 3.