60 Control for Aluminum Production
2. The standard deviation of the temperature has been increasing
for the past week as well. This is statistically an out-of-control
signal because its 7/8 points are increasing in a row.
3. The entire distribution of temperature has been skewed to lower
temperatures (middle 50% box of pots less than the long-term
mean) for some weeks before this (box plot top left).
4. From the single pot box and whisker plots, consecutive pots on the
line often form groups that are all low or all high in temperature,
indicating possibly that there is an operational procedure or
group feature like a crew, which may be inuencing the tem-
perature in these groups. Anode setting groups, measurement
groups metals tapping ...