Stats people: How many data points required to construct a control chart?
Types of Control Charts There are four major types of control charts. The first type, the x-chart (and related xbar, r, and s-charts) is a generic and simple control chart. (There are advantages to the xbar, r, and s-charts, and details are available in many statistical texts.) This Appendix, however, will concentrate on the x-chart, which is the easiest to construct. The x-chart is designed to be used primarily with “variables” data, which are usually measurements such as the length of an object, the time it takes to complete a process, or the number of objects produced per period. Examples at DOE facilities may include square meters of contaminated area cleaned per quarter, liters of waste processed, or time to complete a maintenance work order.In addition to the x-chart there are three specialized types of control charts, the p-chart, c-chart, and u-chart. These charts are used when the data being measured meet certain conditions (or attributes). The p-chart is used with “binomial”
teh number of data points for a sufficiently accurate control chart has to be at least twice as many as the number of signal cycles in the given time period. this is described by the nyquist criterion as fs>=2f (sampling frequency must be greater than or equal to the systems frequency). To see how this makes sense, imagine a sinousoidal waveform with a frequency of 2Hz. If i were to sample at 2Hz, i would always detect a peak or a through, and i wouldn’t be able to realise that the signal is actually oscilalting. however if i were to sample the signal at 4Hz or more, i would see an oscillating (albeit unaccurate) signal. Ideally teh sampling frequency is taken to be as large as possible, to allow for greater accuracy….however it should not be too large so that it will diminish your system’s response.
