Simple Random Sampling

The SURVEYSELECT Procedure

Simple Random Sampling

The following PROC SURVEYSELECT statements select a probability sample of customers from the Customers data set using simple random sampling.

   title1 'Customer Satisfaction Survey';
   proc surveyselect data=Customers method=srs n=100
      out=SampleSRS;
   run;

The PROC SURVEYSELECT statement invokes the procedure. The DATA= option names the SAS data set Customers as the input data set from which to select the sample. The METHOD=SRS option specifies simple random sampling as the sample selection method. In simple random sampling, each unit has an equal probability of selection, and sampling is without replacement. Without-replacement sampling means that a unit cannot be selected more than once. The N=100 option specifies a sample size of 100 customers. The OUT= option stores the sample in the SAS data set named SampleSRS.

Figure 63.2 displays the output from PROC SURVEYSELECT, which summarizes the sample selection. A sample of 100 customers is selected from the data set Customers by simple random sampling. With simple random sampling and no stratification in the sample design, the selection probability is the same for all units in the sample. In this sample, the selection probability for each customer equals 0.007423, which is the sample size (100) divided by the population size (13,471). The sampling weight equals 134.71 for each customer in the sample, where the weight is the inverse of the selection probability. If you specify the STATS option, PROC SURVEYSELECT includes the selection probabilities and sampling weights in the output data set. (This information is always included in the output data set for more complex designs.)

The random number seed is 39647. PROC SURVEYSELECT uses this number as the initial seed for random number generation. Since the SEED= option is not specified in the PROC SURVEYSELECT statement, the seed value is obtained using the time of day from the computer's clock. You can specify SEED=39647 to reproduce this sample.

Customer Satisfaction Survey

The SURVEYSELECT Procedure

Selection Method	Simple Random Sampling

Input Data Set	CUSTOMERS
Random Number Seed	39647
Sample Size	100
Selection Probability	0.007423
Sampling Weight	134.71
Output Data Set	SAMPLESRS

Figure 63.2: Sample Selection Summary

The sample of 100 customers is stored in the SAS data set SampleSRS. PROC SURVEYSELECT does not display this output data set. The following PROC PRINT statements display the first 20 observations of SampleSRS.

   title1 'Customer Satisfaction Survey';
   title2 'Sample of 100 Customers, Selected by SRS';
   title3 '(First 20 Observations)';
   proc print data=SampleSRS(obs=20);
   run;

Figure 63.3 displays the first 20 observations of the output data set SampleSRS, which contains the sample of customers. This data set includes all the variables from the DATA= input data set Customers. If you do not want to include all variables, you can use the ID statement to specify which variables to copy from the input data set to the output (sample) data set.

Customer Satisfaction Survey

Sample of 100 Customers, Selected by SRS

(First 20 Observations)

Obs	CustomerID	State	Type	Usage
1	036-89-0212	FL	New	74
2	045-53-3676	AL	New	411
3	050-99-2380	GA	Old	167
4	066-93-5368	AL	Old	1232
5	082-99-9234	FL	New	90
6	097-17-4766	FL	Old	131
7	110-73-1051	FL	Old	102
8	111-91-6424	GA	New	247
9	127-39-4594	GA	New	61
10	162-50-3866	FL	New	100
11	162-56-1370	FL	New	224
12	167-21-6808	SC	New	60
13	168-02-5189	AL	Old	7553
14	174-07-8711	FL	New	284
15	187-03-7510	SC	New	21
16	190-78-5019	GA	New	185
17	200-75-0054	GA	New	224
18	201-14-1003	GA	Old	3437
19	207-15-7701	GA	Old	24
20	211-14-1373	AL	Old	88

Figure 63.3: Customer Sample (First 20 Observations)

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