Policy for Storing and Reporting Significant Figures for Chemical Data In Reply Refer To: February 14, 2002 Mail Stop 412 Office of Water Quality Technical Memorandum 2002.11 Subject: Policy for Storing and Reporting Significant Figures for Chemical Data This memo establishes the policy of the Office of Water Quality (OWQ) for storing and reporting the appropriate number of significant figures for chemical data in the U.S. Geological Survey's National Water Information System (NWIS) and describes the systematic implementation of this policy. Concepts presented in this memo were developed over the past year by members of the Phoenix Water-Quality User Group, and by staff from OWQ, the Branch of Quality Systems (BQS), and the National Water Quality Laboratory (NWQL). Questions about the established policy and the concepts presented in this memo should be directed to Pete Rogerson (rogerson@usgs.gov) or Stephen Sorenson (sorenson@usgs.gov) in OWQ. Background and Purpose: All water-quality data have an associated uncertainty resulting from variability in sample collection, preparation, and analysis. This memo addresses only the uncertainties associated with laboratory sample preparation and analysis. Data should be stored in and reported from NWIS at a level of significance that accurately reflects these uncertainties. Too many digits imply a higher level of precision than is justified by a particular analytical method. Too few digits potentially eliminate real information about the actual measured constituent concentration that may be critical for interpretation. Current guidance (Novak, 1985, and Hansen, 1991) for reporting significant figures in USGS data reports and other publications recommends that when "presenting numerical data, give only those digits that convey actual information. The last digit should represent the uncertainty in the data." There is no universally recognized means for determining the location of this uncertain digit and the OWQ has not had a defined policy. This memo establishes the practice that will be implemented by the OWQ to derive the significant figures for analytical values by determining the variability of the method using laboratory replicate samples distributed over a range of constituent concentrations. These determinations of variability will then be used to estimate the precision of individual results over the full analytical range of the method. Policy: The convention for reporting rounded analytical values in NWIS is to include all digits known with certainty, plus one digit that is uncertain. The uncertain digit will be called the least significant digit (LSD) in this document. Determination of the LSD will follow the guidance outlined by the American Society for Testing and Materials (1993). Laboratories will report data to the NWIS database to the least significant digit plus one additional digit (LSD+1). One additional uncertain digit is recommended because there is potentially useful information in this added data that will contribute to the ability to statistically evaluate large data sets. Standard USGS publication policy is to report all numbers rounded to the LSD. Standard rounding procedures in NWIS will be written to properly round results to the LSD, but additional retrieval options will be available for users to retrieve data at the LSD+1 level for use in statistical and interpretive analyses. Measurement of variability in analytical methods and the designation of significant figures for each measured constituent is the responsibility of each laboratory providing data to NWIS. The number of significant figures to be reported for a particular laboratory analysis by a particular method will be based on measurement variability for multiple points in the concentration range. The performance of analytical methods will continue to be monitored by the individual laboratories to determine if the initial variability determined for the method is representative of longer-term variability. Each laboratory must define the rationale and operational procedures they will use to report significant figures. A review of these procedures will become part of periodic reviews of each laboratory by the BQS, as implementation of this policy is achieved. Implementation: It will take time to implement the substantial changes outlined in this memo. Structural changes and changes in reporting conventions will be required for NWIS and upgrades will need to be made to laboratory software. The OWQ and BQS will initially pilot this change with the NWQL and the Ocala Water Quality and Research Laboratory (OWQRL). Following successful implementation, the plan will be applied to field water-quality determinations and at other water-quality production laboratories. Currently the NWIS database does not have the capability to store the necessary information to fully implement this policy, and will not have this capability until the 4_2 release currently scheduled for FY 2002. The Phoenix Water-Quality User Group, OWQ, and BQS will continue to work with the NWQL and OWQRL to develop and test procedures needed for this new policy. Until this policy is fully implemented, data publication and rounding will continue to be done by the default rounding rules provided through the NWIS parameter code dictionary. The guidance provided in this memo does not fit all water-quality data that the USGS uses in its investigations. Some procedures, such as radiochemical determinations, isotope analyses, and counts of organisms in biological samples may not allow for the determination of variability or may produce results that are expressed in units of measurement that are not consistent with this policy. The OWQ and others will be working toward more consistent and scientifically defensible ways to determine how to best present and store these types of data as experience is gained with this process. Regardless of how the LSD is determined, it is critical that every data user decide whether the number of significant figures provided is appropriate for the intended use of the data. References: American Society for Testing and Materials, 1993, Standard practice for using significant digits in test data to determine conformance to specifications: Section 7.4, Reporting Test Results: Annual Book of ASTM Standards, Volume 14.02, pp. 19-22. Hansen, W.R., 1991, Suggestions to authors of the reports of the United States Geological Survey, (7th ed.) pp. 119-121. Novak, C.E., 1985, Preparation of water-resources data reports: U.S. Geological Survey Open File Report 85-480, 331 p. Stephen K. Sorenson /s/ Acting Chief, Office of Water Quality This memorandum does not supersede any other Office of Water Quality Technical Memorandum. Distribution: All WRD Employees ********************************* Stephen K. Sorenson Acting Chief, Office of Water Quality U.S. Geological Survey 412 National Center Reston, VA 20192 703/648-6864 sorenson@usgs.gov