While 3-sigma control chart in quality-control procedure takes into account only type-I error, sequential sampling plan and cumulative-sum chart are derived from the sequential analysis, developed by Wald's criteria using likelihood-ratio and both the type-I error and the type-II error. The viewpoint of hypothesis-test procedure in rejecting defective lots is introduced into the investigation of the sequential sampling inspection and the CUSUM chart. V-mask is useful to detect the change of manufacturing process with smaller samples than the samples employed in the 3-sigma control chart.