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Explore the World of Soft…e: Engineering & Science
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Explore_the_World_of_Software_Engineering_and_Science_HRS_Software_1998.iso
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SPC_GEN.HYP
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1993-02-28
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run1
WHAT IS A RUN?
RUNS - This is any sequence of points for which a large
percentage fall on the same side of the center line. This
definition is vague since many experts disagree on what
conditions to use. The following criteria are used in
SPC EXpert to identify a freak pattern:
(a) More than N points in a row on the same side of
the center line - where N is provided by the user.
The default value for N is 7.
@
run2
WHY IS A RUN UNUSUAL?
1. A chart which is in control should have points randomly scattered
on both sides of the center line.
2. A run indicates that some non-random or unnatural causes of
variation are affecting the process.
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freak_point1
WHAT IS A FREAK POINT?
A freak point is any point on the control chart which
lies outside of the Upper or Lower (+- 3 sigma) control
limits.
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freak_point2
WHY IS A FREAK POINT UNUSUAL?
1. 99.9% of all points in a chart which is in control
should fall within the Upper and Lower control limits.
2. A freak point indicates that some non-random or unnatural
causes of variation are affecting the process.
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freak_point3
WHAT DOES A FREAK POINT MEAN?
1. Freak points mean that something very unusual
has happened in the process.
2. A freak is produced by an outside unusual cause.
3. It isn't present for very long.
4. Freak points are the easiest chart patterns
to find the assignable cause for.
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freak_pattern1
WHAT IS A FREAK PATTERN?
FREAK PATTERN - This is any sequence of points for which a
large percentage fall more than a given amount away from the
center line. This definition is vague since many experts
disagree on what conditions to use. The following criteria
are used in SPC EXpert to identify a freak pattern:
(a) 2 out of 3 points in a row outside of the 2 sigma limits.
(b) 4 out of 5 points in a row outside of the 1 sigma limits.
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freak_pattern2
WHY IS A FREAK PATTERN UNUSUAL?
1. Very few points should fall outside the Warning Limits.
2. If your process is in control, less than 5% of the points
on your chart should fall outside of the 2 sigma (WARNING)
limits. And less than 32% should fall outside of the
1 sigma limits.
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shift_up1
WHAT IS A SHIFT UP?
1. Several consequetive points tending to be below the
average followed by several more consequetive points
tending to be above the average.
2. Something has acted on the process and changed
the distribution.
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shift_up2
WHY IS A SHIFT UP UNUSUAL?
1. A chart which is in control should have points randomly scattered
on both sides of the average.
2. A shift up indicates that some non-random or unnatural causes of
variation are affecting the process.
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shift_up3
WHAT DOES A SHIFT UP MEAN?
1. Something new has been introduced into the process.
2. Something different from what you've been doing or using before.
3. This new element changes the whole process. At least for awhile.
4. You have shifted the center of the process distribution.
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shift_down1
WHAT IS A SHIFT DOWN?
1. Several consequetive points tending to be above the
average followed by several more consequetive points
tending to be below the average.
2. Something has acted on the process and changed
the distribution.
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shift_down2
WHY IS A SHIFT DOWN UNUSUAL?
1. A chart which is in control should have points randomly scattered
on both sides of the average.
2. A shift indicates that some non-random or unnatural causes of
variation are affecting the process.
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shift_down3
WHAT DOES A SHIFT DOWN MEAN?
1. Something new has been introduced into the process.
2. Something different from what you've been doing or using before.
3. This new element changes the whole process. At least for awhile.
4. You have shifted the center of the process distribution.
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cyclic1
WHAT IS A CYCLE?
1. A sequence of several consecutive points which tend
over time to regularly rise and fall.
2. A cycle may have several deviations from this pattern;
however, the overall tendency is for a regular rise
and fall pattern.
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cyclic2
WHY IS A CYCLE UNUSUAL?
1. A chart which is in control should have points which change
randomly with no particular tendencies.
2. A cycle indicates that some non-random or unnatural causes
of variation are affecting the process.
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cyclic4
CYCLIC PATTERN
A cyclic pattern is caused by the same thing happening
at the same time over and over again. Common causes
of cyclic patterns include:
1. Defective equipment
2. Fluctuating electricity
3. Weather conditions such as humidity, temperature,
and pressure
4. Human Factors
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cyclic5
CYCLIC PATTERNS & THE HUMAN FACTOR
1. The most common cause of cyclic patterns is the human
factor.
2. Do not forget to look at the human factor when looking
for causes.
3. Shift changes and new hires show up on the charts.
4. Being tired or unfamiliar with with job can cause some
pretty strange chart patterns.
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stratification1
WHAT IS STRATIFICATION?
STRATIFICATION - Sometimes referred to as "hugging the center
line." This is any sequence of points for which a large percentage
fall less than a given amount away from the center line. This
definition is vague since many experts disagree on what
conditions to use. The following criteria is used in SPC EXpert
to identify a stratification pattern:
(a) N or more points in a row which are within the 1 sigma
limits - where N is provided by the user. The default
value for N is 10.
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stratification2
WHY IS STRATIFICATION UNUSUAL?
1. The points are unnaturally quiet compared to the process
variability found when calculating the control limits.
2. If the process is in control the points should be more spread
out. In fact, you should expect over 31% of your points to
randomly fall above or below the 1 sigma levels.
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stratification3
GENERAL CAUSES OF STRATIFICATION CAN BE GOOD OR BAD
1. A common BAD cause of stratification patterns is errors in
measurement - either
(a) Inadvertant errors in measurement such as switching
to a less precise measuring device, incorrectly
rounding off results, or a change in the definition
of what might constitute a defect/defective.
(b) Intentional errors such as "adjusting" measurements
to be closer to the target (nominal) value or ignoring
a certain class of defects/defectives during inspection.
2. A GOOD cause of a stratification pattern could be that the
natural variability in the process has been suddenly reduced
by some outside assignable factors. If this is the case, you
need to discover what happened to reduce the variability in your
process and continue doing that. Look for a change that
occured in the process somewher during or before the points on
the stratification pattern.
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mixture1
WHAT IS A MIXTURE?
MIXTURE - This is any sequence of points for which a large percentage
fall more than a given amount away from the center line. This
definition is vague since many experts disagree on what
conditions to use. The following criteria are used in SPC EXpert
to identify a mixture pattern:
(a) N or more points in a row which are beyond the 1 sigma
limits around the center line - where N can be set by the user.
The default value for N is 10.
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mixture2
WHY IS A MIXTURE UNUSUAL?
1. The points are unnaturally distant from the center line.
2. There is too much variation about the center line.
3. If the process is in control, more points (about 68%) should
be within 1 standard deviation of the center line.
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mixture3
CAUSES OF MIXTURE PATTERNS
1. A mixture pattern is generally occurs when you sample from
two different distributions.
2. For example, a MIXTURE pattern can occur when you sample from
two different machines or processes and "mix" the measurements
into one control chart.
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inc_trend1
WHAT IS AN INCREASING TREND?
1. A sequence of N or more consecutive points which tend
over time to rise - where N can be defined by the user.
The default value for N is 7.
2. An increasing trend may have several downward steps;
however, the overall tendency is for upward steps.
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inc_trend2
WHY IS AN INCREASING TREND UNUSUAL?
1. A chart which is in control should have points which change
randomly - both upwards and downwards - with no particular
tendencies.
2. An increasing trend indicates that some non-random or unnatural
causes of variation are affecting the process.
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dec_trend1
WHAT IS A DECREASING TREND?
1. A sequence of N or more consecutive points which tend
over time to fall - where N can be defined by the user.
The default value for N is 7.
2. An decreasing trend may have several upward steps;
however, the overall tendency is for downward steps.
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dec_trend2
WHY IS A DECREASING TREND UNUSUAL?
1. A chart which is in control should have points which change
randomly on both upwards and downwards with no particular
tendencies.
2. A decreasing trend indicates that some non-random or unnatural
causes of variation are affecting the process.
@