Abstract: The Control Chart statistical methods, developed by Showhart for the control of quality of manufactured products, are applied to the control of Geiger-Miller counting instruments. Experiments are reported to show the use to the Control Chart method for detecting disturbances in instrumental behavior and for detecting radioactive effects so weak that they are near the limit of detection of the instruments. As a corollary, the control chart can be used to reduce to its practical limit the time required for tests.