CUSUM plots for two surgeons (superimposed for the first 11 cases). Primary SPRT limit line is crossed after 8 consecutive, positively identified SLN and alternate line is crossed after 12. Parameters for the primary line are from Table 3. and are similar for the alternate line except for a type 2 error of 0.1 instead of 0.2. That part of the plot beyond either lower SPRT limit line is statistically meaningless but is left in place to allow for drawing of other hypothetical limit lines. In practice, the plot may be restarted at 0 after the SPRT line is crossed, for the purpose of process monitoring.
East et al. BMC Surgery 2009 9:2 doi:10.1186/1471-2482-9-2