Check sheets are used for data collection (Ishikawa, 1991). They are typi- cally used to collect failure occurrence rates. For example, a 100% inspec- tion may use a check sheet to record all inspection results. This information is helpful for determining the actual failure rate. There are different types of check sheets that can be used.
Check Sheet with Tally Marks
A check sheet (Figure 10.3) should have at a minimum three columns: the defect, the number of occurrences, and the total. Care must be used when selecting the defect categories; the defect types should not be so general that multiple types of defects could be entered under the same category, thereby resulting in confusion. The defect categories must also be clear enough so that everybody using the list can assign the defect found to the correct defect category. It may be helpful to have “other” listed as a defect for defects that do not fit into any other category. However, the defects under other should be described to identify them. They may also need their own defect category if they occur too often.
Each time a defect is found, a tally mark is to be entered across from the appropriate defect name and under the heading “number of occurrences.”
Typically, the first through fourth occurrences are listed as individual hash marks, and the fifth is a mark across the first four, such as ////. Each group of five is separated by an empty space to make counting easier.
Tally marks can also be created using dots at the corners of a box to represent one through four and lines connecting the dots to represent five through eight. Nine and ten would be represented by lines crossing the box (Tukey, 1977). Another alternative method to the traditional tally mark is to use a vertical line for the number one, a horizontal line for the number two, a second horizontal line for the number three and a second vertical line for the number four. A diagonal line is used for the number five, and the process is started over for the next group (Figure 10.4). The advantage of this method is that it is faster to add the tally marks and there is less chance of miscounting the groupings.
1 = 2 = 3 = 4 = 5 = FIGURE 10.4 Tally marks.
Defect Number of Occurrences Total
FIGURE 10.3
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Key Points
• Check sheets are used for the collection of failure data. • The data can be used as the basis for a Pareto diagram.
• The defect categories should be clear enough that different defect types are not inadvertently mixed together and multiple users will constantly assign defects to the appropriate types.
eXAMPLe 10.2
Figure 10.5 shows the use of a check sheet example.
PRoceDURe
Step 1: Make a table containing the following headings: defect, number of occurrences, and total.
Step 2: List the potential defects.
Step 3: Use tally marks to denote each defect item found under each category.
Step 4: Count the groups of tally marks to find the total number of occurrences.
Check Sheet with Graphical Representations
Check sheets with graphical representations are used for data collection during an RCA. It can be effectively used to narrow the actual spatial loca- tion of a problem. A floor plan, photo, or other graphical representation is used, and an X or other symbol is placed where an item or occurrence of interest is found. A symbol key should be provided if multiple symbols are used, such as when many factors are under investigation.
Defect Number of Occurrences Total
Too short //// //// //// // 17
Too long //// / 6
Dented //// 5
Other /// 3
FIGURE 10.5
Key Points
• Check sheets with graphical representations can be used with maps, sketches of an area, process flowcharts, or other graphical represen- tations of an area.
• They can also be used with photos of the item in question.
eXAMPLe 10.3
A hypothetical manufacturing company has a problem with bolts being dropped into a machine on an assembly line. Unfortunately, the bolt size in question is used at multiple workstations, so the exact location of the prob- lem is unknown. Extra bolts are frequently found in both finished product at the final inspection and in the dustpans of the cleaning crew, which indi- cates that some bolts drop into the product and others fall to the floor.
A quality engineer draws a schematic sketch of the assembly line and performs a twice- daily floor check to find dropped bolts. Each bolt found is indicated by an X on the sketch of the production floor (Figure 10.6).
PRoceDURe
Step 1: Create a map, flowchart, photo, or diagram of the entire area under investigation.
Step 2: Determine the time frame for the data collection, for example, every hour or at the end of a shift, day, or week.
Step 3: Check for the factor under investigation.
Step 4: A mark should be placed on the graphical representation where the factor under investigation is found.
Connect
plate bracketMount Connect
wiring
Close
housing sub-assemblyInstall Tighten
bolts Direction of
process flow: during shift: XBolts found
X X X X X X FIGURE 10.6
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