SPSS Programming and Data Management, 4th Edition: A Guide for ...

17Best Practices and Efficiency Tips• Use /* and */ to set off a comment within a command.• The closing */ is optional when the comment is at the end of the line. The commandcan continue onto the next line just as if the inserted comment were a blank.• To ensure that comments that refer to macros by name don’t accidently invokethose macros, use the /* [comment text] */ format.Using SET SEED to Reproduce Random Samples or ValuesWhen doing research involving random numbers—for example, when randomlyassigning cases to experimental treatment groups—you should explicitly set therandom number seed value if you want to be able to reproduce the same results.The random number generator is used by the SAMPLE command to generate randomsamples and is used by many distribution functions (for example, NORMAL, UNIFORM)to generate distributions of random numbers. The generator begins with a seed,alargeinteger. Starting with the same seed, the system will repeatedly produce the samesequence of numbers and will select the same sample from a given data file. At thestart of each session, the seed is set to a value that may vary or may be fixed, dependingon your current settings. The seed value changes each time a series of transformationscontains one or more commands that use the random number generator.ExampleTo repeat the same random distribution within a session or in subsequent sessions, useSET SEED before each series of transformations that use the random number generatorto explicitly set the seed value to a constant value.*set_seed.sps.GET FILE = 'c:\examples\data\onevar.sav'.SET SEED = 123456789.SAMPLE .1.LIST.GET FILE = 'c:\examples\data\onevar.sav'.SET SEED = 123456789.SAMPLE .1.LIST.• Before the first sample is taken the first time, the seed value is explicitly set withSET SEED.• The LIST command causes the data to be read and the random number generatorto be invoked once for each original case. The result is an updated seed value.