Evidence Evidence

Human Research Program Evidence

The NASA Human Research Program (HRP) Evidence is a collection of evidence-based risk reports or cited journal articles for each individual risk contained within the HRP Program Requirements Document (PRD). Thus, this set of reports provides the current record of the state of knowledge from research and operations for each of the defined human health and performance risks for future NASA exploration missions. The Evidence Reports provide a brief review article containing the evidence related to a specified risk, written at a level appropriate for the scientifically-educated, non-specialist reader. (For more information regarding the overview and evolution of the evidence, see
Evidence Book Overview.)


A. Evolution of the Evidence Book

The original Evidence Book is a collection of Evidence Reports created from the information presented verbally and discussed within the HRP in 2006. In April of 2008, the 2008 Evidence Book was reviewed by the members of the Committee on NASA's Research on Human Health Risks, established by the Institute of Medicine (IOM). The resulting thorough Review of NASA's Human Research Program Evidence Books: A Letter Report (2008) provided guidance for both the revision of the current risk Evidence Reports and for the development of future versions. It is publicly available via the
National Academies Press website.

Per the recommendations of the IOM Review, the Evidence Report information was made publicly available. The methods used to disseminate the information were the publication of the content of selected reports in multiple specialized journals and the publication of a subset of the reports in a collection, forming a NASA Special Publication entitled the HRP Evidence Book 2008. The specialized journal publications containing the Evidence Report information were revised and reviewed per the specifications of the particular journals in which they were published.

Since this original publication, the HRP Elements have continued to update each risk Evidence Report based on results from ongoing research and technology development (R&TD) activities. The HRR contains the most up-to-date Evidence Reports in the above table.

HRP Evidence Book

An archived publication entitled Human Health and Performance Risks of Space Exploration Missions, issued in 2009, is available for reference. Download a copy by clicking this link.

B. Spaceflight and Ground-Based Evidence

Each risk Evidence Report contains a narrative discussion of the risk and its supporting evidence. All cited publicly-available references are listed at the end of the report. In addition, data that are significant or pivotal are summarized in text, tables, and charts in sufficient detail to allow the reader to critique the data and draw conclusions. The authors also indicate whether the data are from human, animal, or tissue, cellular, or molecular studies. The reports discuss evidence from both spaceflight (including biomedical research, Medical Requirements Integration Document [MRID] data, and operational performance or clinical observations) and ground (including space analog research and non-space analog biomedical or clinical research) research. When providing evidence from ground-based studies, authors discuss why these results are likely to be applicable in the space environment, offering any available validation information for the use of these ground-based systems.

C. Categories of Evidence

To help characterize the type of evidence provided in the reports, authors are encouraged to label evidence according to the "NASA Categories of Evidence". These categories indicate whether data are from two possible types of controlled experiments, are observational, or are expert opinion. As shown below, the NASA categories are compared with a more familiar version of a scale for levels of evidence. The use of a coordinated data categorization system is new to many NASA life scientists, but authors are encouraged to use such a system to help clarify the type of evidence presented and thus provide some additional information about the strength of interpretations derived from those data. They are not required to use the categorization system hierarchically.



Design Type 

Silagy & Haines Levels of Evidence*

(for comparison only) 

NASA Categories of Evidence 


Ia. Meta-analysis of randomized trials

I. At least one randomized, controlled trial

Ib. At least one randomized trial

IIa. At least one controlled study without randomization

II. At least one controlled study without randomization, including cohort, case-controlled, or subject operating as own control

IIb. At least one other type quasi-experimental study


III. Non-experimental descriptive studies, e.g., comparative correlation, or case studies

III. Non-experimental observations or comparative, correlation, and case, or case-series studies


IV. Expert committee reports or opinions or clinical experiences of respected authorities

IV. Expert committee reports or opinions of respected authorities based on clinical experiences, bench research, or "first principles"

*Source: Silagy C, Haines A. Evidence Based Practice in Primary Care, 2nd ed., London: BMJ Books, 2001.

D. Computer-Based Simulation Information

Mathematical modeling and computer simulation provide another type of information distinct from experimental evidence, observation, and expert opinion that can support decision making, including the identification of risks. In the Evidence Reports, authors present the results of simulations, the types of models used, and the reasoning that supports the acceptance of the modeling and simulation results as valid and appropriate in the situation of interest. Appropriate references to papers or reports describing the types of verification to which models were subjected and the validation methods used are also provided.