Physical Activity Guidelines Advisory Committee Report
Part F. Scientific Literature Search
Methodology
List of Figures
Background
Immediately after HHS Secretary Michael Leavitt announced plans for the
development of federal physical activity guidelines on October 27, 2006, the
Centers for Disease Control and Prevention (CDC) was assigned to support the
Physical Activity Guidelines Advisory Committee's (PAGAC) review of the
scientific literature. Working with an advisory committee, staff of the
Division of Nutrition, Physical Activity and Obesity (DNPAO) at CDC's National
Center for Chronic Disease Prevention and Health Promotion developed a
conceptual framework for the literature search. They also established a process
to systematically abstract published articles and to make these abstracts
readily accessible to PAGAC members and consultants. The product of this effort
is called the Physical Activity Guidelines for Americans Scientific
Database (http://apps.nccd.cdc.gov/
PhysicalActivityGuidelines).
Conceptual Framework
The overall conceptual framework for this project is found in Figure
F.1. The scientific literature review for Physical Activity Guidelines for
Americans was initially organized around 8 health outcome domains of
interest: Cardiorespiratory Health, Metabolic Health, Mental Health,
Musculoskeletal Health, Functional Health, Cancer, All-Cause Mortality, and
Adverse Events. Of particular interest was the relevant scientific literature
that relates 7 characteristics of physical activity (or exposures) to these
health outcomes: intensity, frequency, duration, pattern, type, caloric
expenditure, and volume. Also of interest — as related to these physical
activity "exposures" — are physiologic states and adaptations to physical
activity that may be precursors to the health outcomes listed above.
Research Questions
At least 7 key research questions were used to guide the literature
review and the deliberations of the PAGAC. For each health outcome of
interest:
- Is there sufficient evidence that physical activity is associated
with [Outcome]?
- Is there sufficient evidence to support differing intensities of
physical activity in relation to the association with [Outcome] or
precursors?
- Is there sufficient evidence that the accumulation of multiple short
periods of physical activity is associated with [Outcome] or precursors?
- Is there sufficient evidence of increased risk with physical
activity associated with [Outcome]?
- Is there sufficient evidence that supports a pattern of weekly
regularity (days per week) of physical activity and association with [Outcome]
or precursors?
- Is there sufficient evidence that different modes (types) of
physical activity are (differentially and similarly) associated with
[Outcome]?
- Is there sufficient evidence that a physical activity exposure other
than 30 minutes per day on most, preferably all, days each week is associated
with [Outcome]?
Figure F.1. Physical Activity
Guidelines for Americans: Conceptual Framework for Literature Review
Operational Plan
Following from the conceptual framework, 3 CDC teams were formed to
conduct the literature reviews around 3 key life stages: youth (aged 5 to 19
years), adults (aged 19 to 64 years), and older adults (aged 65 years and
older). All aspects of the literature review (i.e., search strategy development
and execution, review and triage of papers, cataloguing, retrieving, coding,
data entry, quality control, and payment of coders) were managed by the teams.
Two scientists (one senior, one junior) were appointed as co-leads for each
life-stage team, and coders were assigned to the teams based on the review
workload (e.g., more studies were available for adults than for youth). In
addition, a separate team was formed to develop and implement quality control
procedures.
Phase 1 of the literature review process (October 2006 through June
2007) was carried out by conducting systematic searches of the scientific
literature on physical activity and the health outcomes described above. During
this phase, the teams held weekly meetings to discuss issues that members were
encountering and to devise solutions to move the project forward. Issues
included literature search terms, inclusion/exclusion criteria for the
literature search, study quality assessment, abstraction form and quality
control, database/systems issues, abstraction progress, qualifications required
of the coders, certification process and selection of coders, training
sessions/agenda for abstractors, certificates developed and sent to certified
abstractors, retraining issues, termination of abstractors due to production or
quality control problems, development of an operations manual, preparation for
the PAGAC meetings and materials, health outcome tables, timelines, team
reviews, database revisions, subcommittee reviews/updates, and payment of the
coders and scientific advisors.
Phase 2 of the literature review process (July 2007 through March 2008)
began after the first PAGAC meeting June 28-29, 2007, and was guided by the
needs of the PAGAC. During this phase, team members updated the Phase 1
literature review through June 2007 and worked with PAGAC members to obtain
scientific papers that were not abstracted during Phase 1. This process is
described in Appendix F.1.
Literature Review
Working from the literature review conceptual framework, the CDC teams
performed a standardized review of the scientific literature to provide
evidence for the deliberations of the PAGAC.
Searching the MEDLINE Database
The first step of the review process was to gather studies for possible
inclusion in the database, using defined search strategies (Appendix F.2). Search terms were selected for
physical activity and for each identified health outcome: cardiovascular and
respiratory health, metabolic health, musculoskeletal health, cancer,
functional health, mental health, all-cause mortality, and injuries/adverse
events.
Using the Ovid interface, the CDC teams searched the National Library of
Medicine's (NLM) MEDLINE Database using only Medical Subject Headings (MeSH)
major descriptors for the physical activity term set. They used a combination
of MeSH descriptors and text word synonyms to search for the health outcome
terms set. A listing of all MeSH headings used in the search strategies is
included in Appendix F.2.
Three searches were run, and a combination of MeSH headings and text
word synonyms were used to limit retrieval to 3 age groups: youth, adults, and
older adults. To capture any articles not indexed by age, a fourth search was
run, excluding all previous age group retrieval. A fifth search was run,
combining all age groups to capture items indexed to multiple age groups. The
search strategies for the 5 groups are included in Appendix F.2.
Each search was further limited by restricting retrieval to English
language and to articles published after 1994 that dealt with human subjects
and contained abstracts. Finally, the searches excluded 3 publication types
— comments, editorials, and reviews. Search results were stored in Word
files and imported into Reference Manager Database files.
Selecting the Articles
The CDC teams developed specific inclusion and exclusion criteria to
determine whether studies would be eligible for abstraction. They also
developed an inclusion/exclusion coding system that allowed them to classify
references efficiently and accurately for the abstraction process. This process
was divided into 2 phases: Certain studies of physical activity and a
diagnosable health outcome were abstracted during Phase 1; other studies of
physical activity and risk factors for the health outcomes were held for
possible abstraction at a later date (Phase 2), if requested by the PAGAC.
Inclusion and Exclusion Criteria
Articles were considered for inclusion in the review if they met certain
criteria. Similarly, articles with certain criteria were excluded from the
review. Appendix F.3 provides a detailed explanation of the inclusion and
exclusion criteria developed for this review. (Appendix F.3).
Abstracting the Articles
For each scientific article, abstractors recorded the following
information: Overall study design; sample and participant characteristics;
intervention design and duration (if an intervention study); physical activity
exposure(s), including the dose of physical activity provided to participants
or in which they participated; follow-up time period; health outcome(s); and
the most advanced study results. For example, if a study presented an analysis
adjusted for age and presented the same analysis adjusted for age and body mass
index (BMI), the abstractor was instructed to record the age- and BMI-adjusted
results.
Abstractors were hired, trained, and certified to perform all
abstracting duties, and strict quality control procedures were used throughout
the abstraction project. The quality control team checked and corrected 12.5%
of abstracted papers. Abstractors were put on probationary status if they did
not meet quality control standards. Cursory checks of abstractions were
conducted, and subsequent corrections were made by all members of the Physical
Activity Guidelines team at CDC.
A Web-based data entry system was developed to manage all abstracted
studies for this project. This system was modeled after a similar system that
CDC has used to abstract studies for the Guide to Community Preventive
Services, which provides systematic reviews of community-based
interventions. The physical activity Web-based data entry system includes
summary tables of the scientific articles abstracted as part of the literature
review for the Physical Activity Guidelines for Americans. The summary
tables can be accessed at
http://apps.nccd.cdc.gov/PhysicalActivityGuidelines.
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