# Missing values in occ1950
**URL:** https://forum.ipums.org/t/missing-values-in-occ1950/4114
**Category:** CPS
**Created:** [May 3, 2021, 3:39pm UTC](https://forum.ipums.org/t/missing-values-in-occ1950/4114 "2021-05-03T15:39:34Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@Julia\_Ingenfeld](https://forum.ipums.org/u/Julia_Ingenfeld)
#### Post date: [May 3, 2021, 3:39pm UTC](https://forum.ipums.org/t/missing-values-in-occ1950/4114/1 "2021-05-03T15:39:34Z")
Hello,
I’m working with the occ1950 variable, and noticed that some occupational categories are not consistently available. For example, professors and instructors are only available until 2002 and have missing values for later waves. I am wondering what happened to those occupational groups – were they reclassified into different categories?
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### Author:  [@KariWilliams](https://forum.ipums.org/u/KariWilliams)
#### Post date: [May 4, 2021, 3:17pm UTC](https://forum.ipums.org/t/missing-values-in-occ1950/4114/2 "2021-05-04T15:17:53Z")
This is a function of a change in detail in the underlying occupation codes that are harmonized into OCC1950. OCC1950 harmonizes the original codes available in the variable [OCC](https://cps.ipums.org/cps-action/variables/occ#codes_section) (based on Census occupation codes, which are updated about every 10 years); CPS switched from using the [1990-based coding system](https://cps.ipums.org/cps/codes/occ_19922002_codes.shtml) to the [2000-based coding system](https://cps.ipums.org/cps/codes/occ_20032010_codes.shtml) in 2003. The coding system implemented in 2003 does not include specific subject identifiers for instructors, and instead includes only an “other teachers and instructors” category. You can see this in OCC1950 frequencies; the ‘subject not specified’ (029) code under the ‘Professors and Instructors’ category is available in all years and the frequency roughly doubles beginning in 2003 when instructors of specific subjects are no longer identified.
Depending on your specific research application and years of interest, you may be interested in the [OCC1990](https://cps.ipums.org/cps-action/variables/occ1990#codes_section) or [OCC2010](https://cps.ipums.org/cps-action/variables/occ2010#codes_section) variables, which harmonize all occupation codes to schemes derived from the 1990 and 2010 coding schemes respectively.