Basic Analysis and Graphing - SAS

Chapter 9 Bootstrapping 251
Perform a Bootstrap Analysis
Perform a Bootstrap Analysis
To perform a bootstrap analysis, right-click on a report in a platform report window and select Bootstrap.
Specify the number of bootstrap samples and choose whether to use fractional weights or to split the
selected column. For more information, see “Bootstrap Window Options” on page 251. Every statistic in the
report is bootstrapped.
The bootstrapping process samples from the original data. An invisible data table is used to store the
sampled data and perform the analysis.
Bootstrap Window Options
After you right-click a column in a report and select Bootstrap, the Bootstrapping window appears with the
following options:
Number of Bootstrap Samples The number of times that you want to re-sample the data and compute
the statistics. A higher number results in more precise estimates of the statistics’ properties.
Fractional Weights Uses fractional weights in the re-sampling process.
If Fractional Weights is not selected, the following observations are true:
– The sampling weights assigned to the original observations are integers.
– The influence of an observation is determined by the relative weights. For example, if an observation
is assigned a weight of 2, then that observation appears twice in the sample for the current bootstrap
iteration.
– The sampling weights can be zero. Observations with a weight of zero are not included in the
estimation process for the current bootstrap iteration.
If Fractional Weights is selected, the following observations are true:
– The sampling weights can be non-integers. The influence of an observation is determined by the
relative weights.
– The sampling weights cannot be zero. All of the observations are included in the bootstrap process.
In both cases, the sum of the weights is n (the number of observations in the original data).
Split Selected Column Splits a stacked column of bootstrap results into separate columns, one for each
statistic. For example, say that the column that you right-clicked before choosing Bootstrap has three
statistics in it. In addition to the main results table with all the statistics stacked, another results table is
created with the results for that column, unstacked into separate columns. For an example, see “Stacked
Results Table” on page 251.
Stacked Results Table
The initial results of a bootstrap analysis appear in a stacked results table (Figure 9.5).