The SET CPU_SAMPLING command causes PCA to collect PC values
    in the same way that SET PC_SAMPLING does, with one important
    exception. The CPU sampling interval is based on virtual process
    or CPU time, not system time. When you issue the SET CPU_SAMPLING
    command, PCA will only collect PC values whenever there has been
    one or more clock ticks on the process's clock.

    There are many external factors that can affect the behavior
    of a program, in relation to the system (or wall) clock. For
    example, page faulting and system service wait time (including
    I/O wait time). These conditions make it difficult to determine
    whether the program counter contains a specific location because
    of the structure of the program's algorithm or because of other
    operations occuring in that interval. Under these conditions,
    sampling the PC values based on the process's CPU time would be
    more effective and reproducible, because the effects caused by
    contending processes are reduced.

    You must collect stack PC information if you want to use a /MAIN_
    IMAGE, /CUMULATIVE or /STACK_DEPTH qualifier with the PLOT or TAB
    command in the Analyzer.