This document discusses secondary analysis of existing data sets. It describes publicly available data from sources like the CDC that can be downloaded for free. It also covers privately held data from previous studies that researchers may collaborate on or obtain approval to analyze. The advantages are reduced costs and resources compared to primary data collection, but limitations include working only with existing variables and needing to trust the original data collection methods. Ethics review is typically required for private or clinical data but not anonymous public sources.

2. Chapter 23
Secondary Studies:
Existing Data Sets

3. Overview
• Secondary analysis: the researcher conducting the
statistical analysis has not had (and does not have)
any contact with the individuals whose data are being
examined.
• Sources:
– publicly available individual-level or population-
level data
– privately held survey data
– clinical records

4. Publicly Available Data
• Sharing data is a cost-efficient way to extract as much
information as possible out of data sets.
• For example, the U.S. Centers for Disease Control
and Prevention (CDC) provides, on its website, data
from several nationwide cross-sectional studies:
– National Health and Nutrition Examination Survey
(NHANES)
– National Health Interview Survey (NHIS)
– Behavioral Risk Factor Surveillance System
(BRFSS)

5. Advantages
• Researchers may be able to download an entire
cleaned data set (and supporting documentation)
immediately and at no cost directly from the website
of the sponsoring organization.
• Secondary analysis is an excellent option for
researchers with strong statistical skills but limited
time and/or data collection resources.

6. Limitations
• The analyst is limited to exploring only the topics
included in the original survey.
• The analyst has to trust that the data were collected
using valid and standardized methods.
• It may be difficult to find someone who can answer
questions about the data collection process.
• There is a risk of duplicating the analysis that
someone else has done or is doing.

7. Private Data
• Individual researchers and research teams, such as
those at universities or university hospitals, may have
data available that have not yet been analyzed.
• The original researcher – the one who collected the
data – may be open to a new researcher taking the
lead on analyzing that portion of the data set and
writing up the results for possible publication.
• Limitation: Patient records are often incomplete.

8. Clinical Records
• Clinical records are a common source of data for case
series.
• Individuals working in clinical settings commonly
have access to patient records for research purposes,
provided that the research project receives all
required approvals and will not violate any law (such
as HIPAA) or policy.

9. Ethics Committee Review
• Additional approval by an ethics committee at the
institution where the secondary analysis will be
conducted is usually not required for anonymized
publicly available data.
• Secondary analysis of private data or hospital records
almost always requires review by a research ethics
committee.