SAMPLE procedure

Samples from a set of units, possibly stratified by factors (P.W. Lane).

Options

SEED = scalar	Seed for the random number generator; default 0 i.e. continue from previous generation
NVALUES = scalar	Number of units from which a simple sample is to be taken; default * i.e. as defined by UNITS statement

Parameters

NSAMPLE = scalars or tables	Number of values in simple sample, or table of numbers of values at each combination of levels of its classifying factors; no default
SAMPLE = identifiers	Structure to store the result; no default

Description

Procedure SAMPLE produces a random sample from a set of units. A simple sample can be obtained by setting the NSAMPLE parameter to the required number in the sample, and the NVALUES option to the number of units in the set. The NVALUES option can be omitted if the required number of units has been defined by a UNITS statement earlier in the job.

For a stratified sample, the NSAMPLE option should be set to a table containing the required number of units to be sampled at each combination of levels of the factors classifying the table. The NVALUES option is not then relevant as the set of units is determined by the values of the classifying factors.

The SAMPLE parameter must be set to an identifier, which will be formed into a variate containing a set of NSAMPLE integers in the range (1…NVALUES), obtained by random sampling without replacement. The SEED option can be set to define a starting value for the random numbers used to select the units. This can be omitted if some random numbers have already been generated during the current job; SAMPLE will then take the numbers that continue the previous sequence.

Options: SEED, NVALUES.

Parameters: NSAMPLE, SAMPLE.

Method

For a simple sample, a full set of units (1…NVALUES) is randomly ordered and the first NSAMPLE values are taken. For a stratified sample, the units are sorted according to levels of the classifying factors (after random ordering) and then the requested number of values are taken for each combination of levels.

Action with RESTRICT

The factors classifying the table must not be restricted. The procedure cannot be used on a restricted set of units.

See also

Directive: CALCULATE.

Procedures: GREJECTIONSAMPLE, GRMULTINORMAL, SVSAMPLE.

Functions: GRBETA, GRBINOMIAL, GRCHISQUARE, GRF, GRGAMMA, GRHYPERGEOMETRIC, GRLOGNORMAL, GRNORMAL, GRPOISSON, GRSAMPLE, GRSELECT, GRT, GRUNIFORM.

Commands for: Calculations and manipulation.

Example

CAPTION  'SAMPLE example',\ 
         '1) select a random sample of 10 out of 100 units;'; STYLE=meta,plain
SAMPLE   [SEED=55326; NVALUES=100] NSAMPLE=10; SAMPLE=Selected
PRINT    Selected; DECIMALS=0
CAPTION  !t('2) select specified numbers of units at each combination',\
         'of levels of two factors.')
FACTOR   [LEVELS=3; VALUES=12(1...3)] F1
&        [LEVELS=2; VALUES=6(1,2)3] F2
TABLE    [CLASSIFICATION=F1,F2; VALUES=(1,2)3] Numbers
SAMPLE   NSAMPLE=Numbers; SAMPLE=Chosen
CAPTION  'Show which units and factor combinations have been selected.'
VARIATE  [VALUES=1...36] Unit
RESTRICT Unit,F1,F2; EXPAND(Chosen; 36)
PRINT    Unit,F1,F2; DECIMALS=0
CAPTION  'Demonstrate that the correct numbers of units have been chosen.'
TABULATE [CLASSIFICATION=F1,F2; COUNT=Check]
PRINT    Numbers,Check; DECIMALS=0