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13.3 SAMPLE

     SAMPLE num1 [FROM num2].

SAMPLE randomly samples a proportion of the cases in the active file. Unless it follows TEMPORARY, it operates as a transformation, permanently removing cases from the active dataset.

The proportion to sample can be expressed as a single number between 0 and 1. If k is the number specified, and N is the number of currently-selected cases in the active dataset, then after SAMPLE k., approximately k*N cases will be selected.

The proportion to sample can also be specified in the style SAMPLE m FROM N. With this style, cases are selected as follows:

  1. If N is equal to the number of currently-selected cases in the active dataset, exactly m cases will be selected.
  2. If N is greater than the number of currently-selected cases in the active dataset, an equivalent proportion of cases will be selected.
  3. If N is less than the number of currently-selected cases in the active, exactly m cases will be selected from the first N cases in the active dataset.

SAMPLE and SELECT IF are performed in the order specified by the syntax file.

SAMPLE is always performed before N OF CASES, regardless of ordering in the syntax file (see N OF CASES).

The same values for SAMPLE may result in different samples. To obtain the same sample, use the SET command to set the random number seed to the same value before each SAMPLE. Different samples may still result when the file is processed on systems with differing endianness or floating-point formats. By default, the random number seed is based on the system time.