Measured values of a process characteristic is plotted over time to determine if the process remains in statistical control 1) Control charts for variables Measures a continuous quantity e.g. strength of material C-bar chart and r(ange) chart 2) Control chats for attributes Measures a discrete quantity e.g. dents on a fridge P(roportion) chat and c(ount) chart Less obvious signs - Trends or cyclical patterns in data - Sudden changes in average values - Points consistently near UCL or LCL - 8 consecutive points near UCL or LCL - 6 consecutive points where each is higher than the last