Referring to the SPC chart signals that a process is out of control, what type of problem does each signal indicate? List the signals

What will be an ideal response?

The signals that something has changed are:
1. one or more points outside the control limits
2. nine or more points in a row above or below the center line
3. six or more points moving in the same direction (up or down)
4. 14 points in a row, alternating up and down.

The first signal means that something has caused the process to produce a higher (or lower) measure than would be normally expected. This could be a problem with variability or a jump one way or the other in the process center. Only one such point above or below the control limits is needed to reach the conclusion that a special cause situation exists.

The second signal implies that the process center is drifting up or down. For example, this type of situation could occur in a process that is sensitive to temperature. If for some reason temperature gradually is increasing, the output measure will gradually change with the temperature change.

The third signal indicates that the process average has shifted (higher or lower) due to some special circumstance. For instance, a control setting may have been changed causing the output measures to increase or decrease consistently.

The fourth signal indicates something is causing the process to change between measurements.