The decision time/movement time (DT/MT) measures which are obtained when using reaction-time apparatus incorporating a home key are being used to investigate a wide range of group differences. It is usually assumed that the time from stimulus onset to release of the home key is a good measure of central processing speed. It is argued that it is not always the case since a possible strategy is to release the home key on or just after detecting rather than fully identifying the stimulus, thus giving an invalidly short DT and long MT. Two experiments are described which show subjects do use such a detection strategy when they can. An alternative procedure which masks out stimulus information when the home key is released is shown to make subjects use a full decision strategy and release the home key only after processing sufficient information to respond correctly, thus giving a better measure of cognitive time.