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