ASTM_G_16_-_13.pdf

Designation:G1613Standard Guide forApplying Statistics to Analysis of Corrosion Data1This standard is issued under the fixed designation G16;the number immediately following the designation indicates the year of originaladoption or,in the case of revision,the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This guide covers and presents briefly some generallyaccepted methods of statistical analyses which are useful in theinterpretation of corrosion test results.1.2 This guide does not cover detailed calculations andmethods,but rather covers a range of approaches which havefound application in corrosion testing.1.3 Only those statistical methods that have found wideacceptance in corrosion testing have been considered in thisguide.1.4 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.2.Referenced Documents2.1 ASTM Standards:2E178 Practice for Dealing With Outlying ObservationsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodG46 Guide for Examination and Evaluation of Pitting Cor-rosionIEEE/ASTM SI 10 American National Standard for Use ofthe International System of Units(SI):The Modern MetricSystem3.Significance and Use3.1 Corrosion test results often show more scatter thanmany other types of tests because of a variety of factors,including the fact that minor impurities often play a decisiverole in controlling corrosion rates.Statistical analysis can bevery helpful in allowing investigators to interpret such results,especially in determining when test results differ from oneanother significantly.This can be a difficult task when a varietyof materials are under test,but statistical methods provide arational approach to this problem.3.2 Modern data reduction programs in combination withcomputers have allowed sophisticated statistical analyses ondata sets with relative ease.This capability permits investiga-tors to determine if associations exist between many variablesand,if so,to develop quantitative expressions relating thevariables.3.3 Statistical evaluation is a necessary step in the analysisof results from any procedure which provides quantitativeinformation.This analysis allows confidence intervals to beestimated from the measured results.4.Errors4.1 DistributionsIn the measurement of values associatedwith the corrosion of metals,a variety of factors act to producemeasured values that deviate from expected values for theconditions that are present.Usually the factors which contrib-ute to the error of measured values act in a more or less randomway so that the average of several values approximates theexpected value better than a single measurement.The patternin which data are scattered is called its distribution,and avariety of distributions are seen in corrosion work.4.2 HistogramsA bar graph called a histogram may beused to display the scatter of the data.A histogram isconstructed by dividing the range of data values into equalintervals on the abscissa axis and then placing a bar over eachinterval of a height equal to the number of data points withinthat interval.The number of intervals should be few enough sothat almost all intervals contain at least three points;however,there should be a sufficient number of intervals to facilitatevisualization of the shape and symmetry of the bar heights.Twenty intervals are usually recommended for a histogram.Because so many points are required to construct a histogram,it is unusual to find data sets in corrosion work that lendthemselves to this type of analysis.4.3 Normal DistributionMany statistical techniques arebased on the normal distribution.This distribution is bell-shaped and symmetrical.Use of analysis techniques developedfor the normal distribution on data distributed in anothermanner can lead to grossly erroneous conclusions.Thus,beforeattempting data analysis,the data should either be verified asbeing scattered like a normal distribution,or a transformation1This guide is under the jurisdiction of ASTM Committee G01 on Corrosion ofMetals and is the direct responsibility of Subcommittee G01.05 on LaboratoryCorrosion Tests.Current edition approved Dec.1,2013.Published December 2013.Originallyapproved in 1971.Last previous edition approved in 2010 as G1695(2010).DOI:10.1520/G0016-13.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 should be used to obtain a data set which is approximatelynormally distributed.Transformed data may be analyzed sta-tistically and the results transformed back to give the desiredresults,although t