Range--the difference between the largest and
smallest numbers of the set
Deviation--the amount each element of the
population is away from the mean
Standard error--the average percent of
deviation from the mean
CONTROL CHART INTERPRETATION
Control charts for film processing should be
maintained as long as the process is in operation. For
control charts to be of value, you should remember
the following guidelines:
There must be a continuous analysis of a
statistical nature of photo-processing control
charts.
It is pointless to maintain control charts, then
fail to act when the chart indicates a problem
in the process.
Control charts let you visually determine a
definite processing problem occurring among
random process variations.
There is always a certain amount of natural
variability in a process.
There can be overcontrol, as well as
undercontrol, in precision quality assurance
processing. Overcontrol of a process can be
needlessly expensive.
These guidelines cannot be overemphasized and
should become an integral part of your processing
philosophy. Each process control chart you maintain
can illustrate five possible conditions of that process.
These conditions depend upon variations within that
process. These five conditions are shown in figure
2-15.
A normal pattern exists in all processes that are
operating correctly. This pattern reflects the
variability that cannot be controlled or eliminated
completely. When an out-of-control condition exists,
immediate action should be taken to correct the
problem. When the chart shows a trend in five
successive recordings, this is a good indication that
the process is changing and requires investigation with
possible corrective action. A run is when five
successive recordings appear above or below the mean
line. This also requires investigation and possible
corrective action. A jump may indicate that a problem
exists and requires correction before the process gets
out of control.
Evaluating Process-Monitoring Charts
To be useful, you must be accurate with a control
chart and you must analyze it at least daily.
PLOT PATTERNS.--
Plots, or points, on a
control chart should be thought of as patterns or
groups and not as individual points. It is not enough
to know where a plot is; you must also know how it
got there. In process monitoring, you must be able to
recognize patterns that indicate when a process is
moving toward an out-of-control condition. After a
value is plotted on a chart, the point is connected to
the previous point with a straight line. This provides
a graphic representation of variations in the process
and whether the desired processing conditions are
being maintained.
TRENDS.--
A drift of plotted points either
upward or downward, away from the established
mean, with no sudden change in direction is called a
trend pattern. A trend usually consists of at least five
plotted points. An upward or downward trend usually
indicates over- or underdevelopment, respectively.
The processed images will be either increasing or
decreasing in both density and contrast. A trend that
is gradual may be an indication of too much or too
little replenishment.
JUMPS.--
A plot point that jumps or suddenly
moves away from previously plotted points may be
caused by contamination of the chemistry, improperly
mixed chemicals, or mechanical breakdowns, such as
replenisher systems or thermostats. A jump pattern is
likely to occur after a process has been shut down,
especially if the process has not reached the proper
operating temperature.
RANDOM PATTERN.--
Whereas trends and
jumps must be analyzed to determine their probable
cause and corrective action taken when necessary, a
random pattern within control limits indicates that the
process is in control and is not moving toward an
out-of-control condition. When an in-control random
pattern is maintained, solution strength is probably
normal and no correction is necessary; however, when
2-28

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