Frequently Asked Questions (FAQs)

The following FAQs may be of use to researchers, academics and other data experts in search of quick answers to technical questions on how to use CHIS. For a more general explanation of CHIS, please visit the About CHIS section.

See FAQs and answers by clicking on the following topics: 

• General CHIS Data Information
• Public Use Files
• Local Health Departments
• Data Access Center
• AskCHIS/Data Estimates
• Weights
• Technical Questions

For more nuanced questions, please consult our CHIS Forum.

General CHIS Data Information:

1. If I am interested in a specific topic or survey question, where can I find that information?
2. I know a question was asked in the survey, but I can't find it.
3. I need more information about specific variables and their use. Where can I get more information?
4. What is a source variable? How is this different than any other variable?
5. What is a constructed variable? How is this different than any other variable?
6. Why are there no missing values for most variables?
7. What is imputation?
8. I'm looking at the frequency for a variable and the total doesn't match the survey sample size. Why not?
9. My university/institution says I need IRB approval. Does your IRB approval cover my project?
10. How does a CHIS respondent provide informed consent?
11. How do I cite CHIS in my publication?

1. If I am interested in a specific topic or survey question, where can I find that information?
Answer: The best way to find out about topics that are covered in the questionnaires is to check the Survey Topics List for each survey year. You can also check the questionnaires; every variable that is available in the CHIS data files is based on the information collected via the Questionnaires.

2. I know a question was asked in the survey, but I can't find it.
Answer: The survey is conducted on a continuous basis and data is released in a variety of formats. Not all questions were asked in all years. In addition, there are separate questionnaires for adults, adolescents and children. To see questionnaires for each survey cycle, visit our Questionnaires page in the Survey Design & Methods section.

3. I need more information about specific variables and their use. Where can I get more information?
Answer: The first place to start is the Questionnaire. In addition, the data dictionaries contain informative notes and are available for both Public Use Files (PUFs) and confidential source or Sensitive Data. For all years, the data dictionaries include the raw frequencies for nearly every variable. We also have a constructed variables document available on the PUF website which outlines how the constructed variables were created. Should you continue to have questions after that, let us know and we'll do our best to help you.

4. What is a source variable? How is this different than any other variable?
Answer: Source variables are variables based on a single question asked during the CHIS interview. These include confidential variables, as well as those found in the Public Use Files. Source variables are generally labeled with two letters followed by numbers.

5. What is a constructed variable? How is this different than any other variable?
Answer: Constructed variables are variables that were put together by the CHIS Data Production team or by Westat, Inc., the data collection contractor. Constructed variables are usually based on multiple questions asked during the interview. In general, constructed variables can be identified by their variable names, which are acronyms and/or abbreviations.

6. Why are there no missing values for most variables?
Answer: For nearly every question on the surveys, some respondents do not provide a valid response (for example, don't know, refused, or other responses). For these missing items, values have been imputed. See the imputation documentation available online.

7. What is imputation?
Answer: Variables that are imputed have had the missing values replaced by a value that was generated based on a complicated algorithm. Detailed information about imputation methods can be found here.

8. I'm looking at the frequency for a variable and the total doesn't match the survey sample size…why not?
Answer: Not every question is asked of every respondent. Many questions are limited to a particular group through skip patterns. These skip patterns create a universe of respondents eligible to answer a question. For example, the universe for a question on prostate screening in the past year is limited to 'all adult men age 40 years and older.'?

9. My university says I need IRB approval…does your IRB approval cover my project?
Answer: No. If you who are not affiliated with UCLA, you should consult with your local IRB to determine the appropriate review of your proposed research, but you are not required to submit proof of IRB approval to the DAC. Most likely, you’ll be able to get an “Exemption”, but check with your local IRB, as you may need IRB approval from your institution. If you are affiliated with UCLA, you should NOT submit an application to the UCLA Human Subjects Protection Committee (IRB), as your project is covered by IRB#11-002227, which the UCLA South General IRB has approved to allow the DAC to conduct analyses on confidential CHIS data. Unless you, or anyone on your project team, are CHIS staff with access to confidential CHIS data, in which case you are required to obtain UCLA IRB approval and submit a copy of the approval with your DAC application.

10. How does a CHIS respondent get informed consent?
Answer: As part of the survey, a screener script is read that includes: 1) an introduction of the interviewer, UCLA, and the survey sponsors; 2) an explanation of purpose and importance of the survey; 3) statements describing the confidential and voluntary nature of the survey; and 4) statements explaining the respondent's right to skip any questions and to end the interview at any time.
The (potential) respondent is then asked if he/she wishes to participate in the survey. The respondent implies consent if they choose to continue with the survey.

11. How do I cite CHIS in my publication?
Answer: Click on the following link to learn the quick and easy CHIS Citation here

Public Use Files

1. Why are some items in the survey not in the Public Use Files (PUF)?
2. How can a specific variable be considered identifiable if it's at the state level?
3. How do I link the Adult, Adolescent, and/or Child files?
4. I have never analyzed data before. Can someone help me figure out what this all means?

1. Why are some items in the survey not in the Public Use Files (PUF)?  Answer: Some variables are deemed to be either sensitive or identifiable or both. The data for these variables cannot be disclosed in the Public Use Files.

2. How can a specific variable be considered identifiable if it's at the state level?
Answer: There is a distinction between identifiable and sensitive. Sensitive variables contain information that would likely harm or embarrass the individual if disclosed. Both identifiable and sensitive variables should be excluded from public use files according to the National Center for Health Statistics.

3. How do I link the Adult, Adolescent, and/or Child files?
Answer: As an additional layer of protection for the identity of the respondents, the variable that links the Adult, Adolescent, and Child files has been removed from the PUF. This variable is available only through the Data Access Center. However, some information from the Adult file has been added to the Child and Adolescent records (e.g., race, poverty status).

4. I have never analyzed data before. Can someone help me figure out what this all means?
Answer: We suggest you first try AskCHIS, an easy to use online query system that enables you to analyze data quickly, easily and free of charge. There is also a multitude of supporting documentation on our website, including data dictionaries. We offer limited technical assistance on how variables were constructed and the general structure of the dataset. We will not, however, assist in the interpretation of the data. There are online resources available to help in this regard. 

Local Health Departments

1.I work for a county health department and want to use CHIS data specific to my county. How can I obtain that data?

2.I want some of the confidential variables. How can I get access to this data?


1. I work for a county health department and want to use CHIS data specific to my county. How can I obtain that data?
Answer: Counties can receive a Local Health Department file. This file is essentially the PUF customized for your county or group of counties. Contact Dacchpr@ucla.edu for information about how to receive this file.

2. I want some of the confidential variables. How can I get access to this data?
Answer: Counties can apply for a confidential data file through the Data Access Center. This data file contains identifiable variables but does not contain sensitive variables. Contact Dacchpr@ucla.edu for information about how to receive files with confidential data for your county.

Working with Confidential Data

1. I am a researcher with an academic institution and need access to confidential data. How do I access these files?
2. Who reviews the Data Access Center application, and what are they looking for?
3. Why are there fees for accessing confidential data?
4. After I pay, will the confidential data file be sent to me?
5. What is disclosure review?
6. Why can't I just request the confidential variables I need and ALL of the Public Use Files variables?
7. Once my project has been approved and the customized dataset has been cut, will I be able to add variables to my dataset?
8. Do I have to pay for DAC use if I am a student?
9. What type of software do you support in the DAC?
10. I've never analyzed data before. Can someone help me figure out what this all means?

1. I am a researcher with an academic institution and need access to confidential data. How do I access these files?
Answer: You'll need to apply to use the confidential data through the Data Access Center (DAC). The application process consists of submitting a few forms and some supplementary documentation to the CHIS Data Disclosure Review Committee (DDRC). More information on the DAC, including application materials and variable lists, can be found after logging in to the DAC website located here.

2. Who reviews the Data Access Center application, and what are they looking for?
Answer: The application is reviewed by the Data Disclosure Review Committee (DDRC). This committee consists of representatives from the California Department of Public Health, and the UCLA Center for Health Policy Research. The committee reviews the application to make sure the project: a) is consistent with the purposes of CHIS; b) is feasible; and c) will not threaten the confidentiality of the respondent. Projects that satisfy these requirements are recommended for approval. The CHIS Principal Investigator, Dr. Ninez Ponce, makes the final determination of whether to approve, reject, or suggest revision for a project proposal.

3. Why are there fees for accessing confidential data?
Answer: The fees associated with accessing the data help cover the costs of data production, committee review, providing a customized data file, and initial technical assistance. See the Data Access Center Fact Sheet for more information about fees.

4. After I pay, will the confidential data file be sent to me?
Answer: No, confidential data files do not leave the Data Access Center at UCLA. Data must be analyzed remotely by sending in programming code (SAS, SPSS, SAS-callable SUDAAN, or Stata) or by using programming/consulting services to assist in analysis. Output is returned to you after disclosure review.

5. What is disclosure review?
Answer: We review all output for small (raw frequency) cell sizes and make release decisions accordingly. For example, if you have generated a cross-tab of Pacific Islander women over the age of 50 by Census tract, we will not return that to you because it violates confidentiality and/or sensitivity requirements.

6. Why can't I just request the confidential variables I need and ALL of the
Public Use Files variables?
Answer: A dataset that contains any confidential variable is considered confidential and the more information contained in that dataset, the greater the risk to the respondent's confidentiality. In an effort to reduce that risk as much as possible, we require that customized datasets only contain those variables relevant to a particular project.

7. Once my project has been approved and the customized dataset has been cut, will I be able to add variables to my dataset?
Answer: Yes. We understand that you occasionally may neglect to select particular variables that are important to your project. Variables can be added by requesting the specific variables and providing a brief justification for adding the selected variables. For example, if your project is about the effect of household smoking on asthma and you did not select "age at asthma diagnosis" we will add that variable at your request.

8. Do I have to pay for DAC use if I am a student?
Answer: Yes, but there is a limited scholarship available to help cover part of the DAC fees typically paid by researchers. Contact the Data Access Center Manager at (310) 794-8362 or dacchpr@ucla.edu, for more information.

9. What type of software do you support in the DAC?
Answer: Primarily we support SAS, SPSS, SAS-callable SUDAAN, and Stata. Other software are supported to a more limited extent. Please contact us about your software needs before submitting a DAC application. Even though you may have paid for other software, it cannot be put on our network, which is limited to the software for which we are licensed to use.

10. I've never analyzed data before. Can someone help me figure out what this all means?
Answer: You can first try AskCHIS, an online query system that enables you to analyze data quickly, easily and free of charge. We offer limited technical assistance in regard to understanding how variables are constructed and the general structure of the dataset. For a fee, we offer consultation services through our Data Estimate Services. There is also a multitude of supporting documentation on our website.

AskCHIS/Data Estimates

1. All I really want is a weighted estimate at the county level. Do I still have to go through the Data Access Center?
2. Are all variables in the CHIS questionnaire in AskCHIS?
3. I need some information that I can't get from AskCHIS and still only want the weighted percentages. Where can I find that information?
4. My estimates from AskCHIS are unstable (i.e., they have a high coefficient of variation or wide confidence intervals). Can I use them?
5. How should I report single-year PUF or AskCHIS estimates that are unstable?
6. I ran two-year estimates for 2011-2012 data in the past (before August 2015) and they are different from the pooled 2011 and 2012 pooled estimates I subsequently produced. Why?
7. I ran estimates in the past, and they are different from what I produced from my query today. Why?

1. All I really want is a weighted estimate at the county level. Do I still have to go through the Data Access Center?
Answer: No. You can use AskCHIS, an online query system that enables you to analyze data quickly, easily and free of charge. It contains weighted totals and percentages for nearly all variables in the CHIS datasets. You can get county or multi-county level cross-tabulated estimates.

2. Are all variables in the CHIS questionnaire in AskCHIS?
Answer: No. It is a complicated and time-consuming process to develop variables for AskCHIS. If you have a suggestion for an AskCHIS variable, please send an email suggestion to chis@ucla.edu.

3. I need some information that I can't get from AskCHIS and still only want the weighted percentages. Where can I find that information?
Answer: You can use our Data Estimates Service. This service can, for a fee, provide you with the weighted percentages and population totals at sub-state geographic levels for almost any variable or set of variables. Unreliable estimates (as a result of variables with very small sample sizes) will be suppressed. Please email your interest to chis@ucla.edu. AskCHIS also provides subsetting for a limited set of variables. 

4. My estimates from AskCHIS are unstable (i.e., they havea high coefficient of variation or wide confidence intervals). Can I use them?
Answer: This depends largely on your purpose and how and where you will report or use the estimates. CHIS uses the coefficient of variation (CV) and the confidence interval (CI), both calculated from the standard error of the estimate, to express the sampling variance (or “sampling error”) around an estimate. The calculation of the standard error of the estimate takes into account the complex sample survey design in CHIS.

• The CV indicates whether or not a point estimate (e.g., a mean, proportion, total) is statistically stable relative to its standard error. The CV is a ratio of the standard error to the estimate, showing the proportion of the estimate that reflects sampling variability: CV(?^)=se (?^) / ?^. In AskCHIS, estimates with a CV greater than 30% are "flagged" as statistically unstable with a red asterisk (*). While there are no absolute standards for judging how much variabililty is too much, CHIS generally recommends against reporting or relying on estimates that are this statistically unstable.
  
  
• Users will sometimes use a CI to see if an estimate is "statistically greater" than zero (or some other meaningful number) or if two estimates of subsamples from the same data are different from each other. Such uses are similar to "statistical significance testing" but not identical (Schenker & Gentleman, 2012). AskCHIS and AskCHIS NE report 95% confidence intervals.

Reasonable exceptions to the CV > 30% guideline may apply to binary/dichotomous variables when the magnitude of estimates are very small or very large (i.e., below 10% or above 90%). CHIS calculates the CV for binary variables by using the category with the smallest value as the denominator, which means that the CV will be the same regardless of which estimate you choose to report for the binary indicator of interest. This is a conservative approach that minimizes the potential for reporting unstable estimates. 

The calculation can also produce a high CV even when the sample size is relatively large. For example, AskCHIS shows that 2.6% of 0-5 year olds in California are uninsured and 95.1% have a usual source of care. Both estimates are flagged as statistically unstable even though the sample of this population is large, suggesting that they are not reportable. This is a limitation of the CV, which increases as the denominator approaches 0. Page 10 of Lee et al. (2007) contains a detailed explanation of this issue.

Under such circumstances (dichotomous variables with low or high values) it may be reasonable to report the estimate if the CI range is acceptable to data users, e.g., reporting estimates with confidence limits within +/- 5%.  It may help to consult the statistical standards of your agency, company, or professional association or the standards used in the journals in which you hope to publish. 

If, however, you are in doubt or do not have another standard to use we recommend deferring to the CV flags that caution against reporting estimates with high CVs, unless CI ranges are appropriate for your purpose.

5. How should I report single-year PUF or AskCHIS estimates that are unstable?

Answer: Beginning in 2011, CHIS data has been collected continuously, enabling the release of single-year data files (Public Use Files [PUFs] and AskCHIS) to provide users with timely data.  However, CHIS is designed to provide stable estimates at the county level over a full two-year cycle of data collection. Thus single-year county-level estimates (or estimates for small subgroups) may not be stable. Trying to further analyze a sub-population within county with single-year data may lead to unstable estimates. When this becomes a problem, we strongly recommend that you combine two or more years of CHIS data (e.g., pooling 2013 and 2014) to help stabilize these data. To combine years in AskCHIS, under the “Years” tab choose the years you want to combine and then select the “Pool” option.

 

6. I ran two-year estimates for 2011-2012 data in the past (before August 2015) and they are different from the pooled 2011 and 2012 pooled estimates I subsequently produced. Why?

Answer: With the shift to continuous data collection in 2011, CHIS has developed one-year weights to provide estimates for each year of data collection. In general, estimates obtained by combining multiple years (such as 2011 and 2012) will be close, but not necessarily identical to, estimates produced from a two-year data file (such as 2011-2012) obtained prior to August 2015.

 

Part of this is due to differences in the population control totals used to weight one-year data and two-year data. For both data sets, control totals were created from California Department of Finance’s (DOF) Population Estimates and Population projections (for more information, refer to CHIS Methodology Report 5: Weight and Variance Estimation). The CHIS 2011-2012 two-year weights used the population control totals that represent 2012; since data from respondents were collected over this two-year period, the estimates represent the experience of respondents in this timeframe. The 2011 and 2012 CHIS one-year weights, on the other hand, are representative of the population of California in 2011 and 2012, respectively, and use the population control totals unique to 2011 and 2012. Pooling 2011 and 2012 together will provide a population-weighted average for the 2011-2012 data collection period. 

7. I ran estimates in the past, and they are different from what I produced from my query today. Why?

Answer:  CHIS staff periodically update estimates. When large discrepancies are discovered, we send out Data Advisories to affected users and/or post them on our Forum page. You may contact the Data Access Center (dacchpr@ucla.edu) if you have additional questions.