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Chapter 1
Unequal selection probabilities. When sampling clusters that contain unequal numbers of units,
you can use probability-proportional-to-size (PPS) sampling to make a cluster’s selection
probability equal to the proportion of units it contains. PPS sampling can also use more general
weighting schemes to select units.
Unrestricted sampling. Unrestricted sampling selects units with replacement (WR). Thus, an
individual unit can be selected for the sample more than once.
Sampling weights. Sampling weights are automatically computed while drawing a complex
sample and ideally correspond to the “frequency” that each sampling unit represents in the target
population. Therefore, the sum of the weights over the sample should estimate the population
size. Complex Samples analysis procedures require sampling weights in order to properly analyze
a complex sample. Note that these weights shouldbeusedentirelywithintheComplexSamples
option and should not be used with other analytical procedures via the Weight Cases procedure,
which treats weights as case replications.
Usage of Complex Samples Procedures
Your usage of Complex Samples procedures depends on your particular needs. The primary
types of users are those who:
Plan and carry out surveys according to complex designs, possibly analyzing the sample later.
The primary tool for surveyors is the Sampling Wizard.
Analyze sample data files previously obtained according to complex designs. Before using the
Complex Samples analysis procedures, you may need to use the Analysis Preparation Wizard.
Regardless of which type of user you are, you need to supply design information to Complex
Samples procedures. This information is stored in a plan file for easy reuse.
Plan Files
Aplanfile contains complex sample specifications. There are two types of plan files:
Sampling plan. The specifications given in the Sampling Wizard defineasampledesignthat
is used to draw a complex sample. The sampling plan file contains those specifications. The
sampling plan file also contains a default analysis plan that uses estimation methods suitable for
the specified sample design.
Analysis plan. This plan file contains information needed by Complex Samples analysis procedures
to properly compute variance estimates for a complex sample. The plan includes the sample
structure, estimation methods for each stage, and references to required variables, such as sample
weights. The Analysis Preparation Wizard allows you to create and edit analysis plans.
There are several advantages to saving your specifications in a plan file, including:
A surveyor can specify the first stage of a multistage sampling plan and draw first-stage
units now, collect information on sampling units for the second stage, and then modify the
sampling plan to include the second stage.