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    Basic Tools for Process Improvement

    Module 11

    HISTOGRAM

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    Basic Tools for Process Improvement

    What is a Histogram?

    A Histogram is a vertical bar chart that depicts the distribution of a set of da

    Run Charts or Control Charts, which are discussed in other modules, a Hisdoes not reflect process performance over time. It's helpful to think of a Hisas being like a snapshot, while a Run Chart or Control Chart is more like a

    (Viewgraph 1).

    When should we use a Histogram?When you are unsure what to do with a large set of measurements presenttable, you can use a Histogram to organize and display the data in a more

    friendly format. A Histogram will make it easy to see where the majority of falls in a measurement scale, and how much variation there is. It is helpful construct a Histogram when you want to do the following (Viewgraph 2):

    ! Summarize large data sets graph ically. When you look at Viewgyou can see that a set of data presented in a table isnt easy to use. make it much easier to understand by summarizing it on a tally shee

    (Viewgraph 7) and organizing it into a Histogram (Viewgraph 12).

    ! Compare process results with specification limits. If you add

    process specification limits to your Histogram, you can determine quwhether the current process was able to produce "good" products.

    Specification limits may take the form of length, weight, density, quanmaterials to be delivered, or whatever is important for the product of

    process. Viewgraph 14 shows a Histogram on which the specificatioor "goalposts," have been superimposed. Well look more closely atimplications of specification limits when we discuss Histogram interp

    later in this module.

    ! Communicate information g raphically. The team members cansee the values which occur most frequently. When you use a Histogsummarize large data sets, or to compare measurements to specific

    limits, you are employing a powerful tool for communicating informat

    ! Use a tool to assist in decision making. As you will see as we m

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    HISTOGRAM VIEWG

    What Is a Histogram?

    A bar graph that shows the distribution of data

    A snapshot of data taken from a process

    0 5 10 15 20 25 30 35 40 45 50 55 60

    0

    20

    40

    60

    80

    100

    When Are Histograms Used?

    Summarize large data sets graphically

    Compare measurements to specification

    Communicate information to the team

    Basic Tools for Process Improvement

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    Basic Tools for Process Improvement

    What are the parts of a Histogram?

    As you can see in Viewgraph 3, a Histogram is made up of five parts:

    1. Title: The title briefly describes the information that is contained in t

    Histogram.

    2. Horizontal or X-Axis : The horizontal or X-axis shows you the sca

    values into which the measurements fit. These measurements are ggrouped into intervals to help you summarize large data sets. Individpoints are not displayed.

    3. Bars: The bars have two important characteristicsheight and widt

    height represents the number of times the values within an interval oThe width represents the length of the interval covered by the bar. It

    same for all bars.

    4. Vertical or Y-Axis : The vertical or Y-axis is the scale that shows y

    number of times the values within an interval occurred. The number

    is also referred to as "frequency."

    5. Legend: The legend provides additional information that documents

    the data came from and how the measurements were gathered.

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    HISTOGRAM VIEWG

    1 Title 2 Horizon tal / X-axis

    3 Bars 4 Vertical / Y-axis

    5 Legend

    0 5 10 15 20 25 30 35 40 45 50 55 60

    DAYS OF OPERATION PRIOR TO

    FAILURE FOR AN HF RECEIVER

    DAYS OF OPERATION

    MEAN TIME BETWEEN FAILURE (IN DAYS) FOR R-1051 HF RECEIVER

    Data taken at SIMA, Pearl Harbor, 15 May - 15 July 94

    Parts of a Histogram

    1

    F

    R

    E

    Q

    UE

    N

    C

    Y

    4

    5

    0

    20

    40

    60

    80

    100

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    Basic Tools for Process Improvement

    How is a Histogram constructed?

    There are many different ways to organize data and build Histograms. You

    safely use any of them as long as you follow the basic rules. In this moduleuse the nine-step approach (Viewgraphs 4 and 5) described on the followin

    EXAMPLE: The following scenario will be used as an example to provide

    we go through the process of building a Histogram step by step:

    During sea trials, a ship conducted test firings of its MK 75,76mm gun. The ship fired 135 rounds at a target. An airbornespotter provided accurate rake data to assess the fall of shotboth long and short of the target. The ship computed what

    constituted a hit for the test firing as:

    From 60 yards short of the target

    To 300 yards beyond the target

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    HISTOGRAM VIEWG

    Step 1 - Count number of data point

    Step 2 - Summarize on a tally sheet

    Step 3 - Compute the range

    Step 4 - Determine number of interv

    Step 5 - Compute interval width

    Constructing a Histogram

    Constructing a Histogram

    Step 6 - Determine interval starting

    points

    Step 7 - Count number of points in

    each interval

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    Basic Tools for Process Improvement

    Step 1 - Count the total number of data points you have listed. Sup

    team collected data on the miss distance for the gunnery exercise descthe example. The data you collected was for the fall of shot both long athe target. The data are displayed in Viewgraph 6. Simply counting the

    number of entries in the data set completes this step. In this example, t135 data points.

    Step 2 - Summarize your data on a tally sheet. You need to summariz

    data to make it easy to interpret. You can do this by constructing a tally

    First, identify all the different values found in Viewgraph 6 (-160, -010

    220, etc.). Organize these values from smallest to largest (-180, -12410).

    Then, make a tally mark next to the value every time that value is prethe data set.

    Alternatively, simply count the number of times each value is presendata set and enter that number next to the value, as shown in Viewg

    This tally helped us organize 135 mixed numbers into a ranked sequencvalues. Moreover, we can see very easily the number of times that eacappeared in the data set. This data can be summarized even further by

    intervals of values.

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    HISTOGRAM VIEWG

    Step 1 - Count the total number of data po

    -180 30 190 380 330 140 160 270 10 - 9- 10 30 60 230 90 120 10 50 250 18

    -130 220 170 130 - 50 - 80 180 100 110 20260 190 -100 150 210 140 -130 130 150 37160 180 240 260 - 20 - 80 30 80 240 13210 40 70 - 70 250 360 120 - 60 - 30 20

    50 20 30 280 410 70 - 10 20 130 17140 220 - 40 290 90 100 - 30 340 20 8210 130 350 250 - 20 230 180 130 - 30 21-30 80 270 320 30 240 120 100 20 7

    300 260 20 40 - 20 250 310 40 200 19

    110 -30 50 240 180 50 130 200 280 6260 70 100 140 80 190 100 270 140 8110 130 120 30 70

    TOTAL = 135

    Number of yards long (+ data) and yards short (- data) that a gun crew missed its

    How to Construct a Histogram

    Step 2 - Summarize the data on a tally she

    How to Construct a Histogram

    DATA TALLY DATA TALLY DATA TALLY DATA TALLY DATA TA

    - 180 3 90

    - 130 2 100- 100 2 110

    - 90 5 120

    - 80 6 130

    - 70 3 140

    - 60 4 150

    1

    21

    1

    2

    1

    1

    - 20

    - 1010

    20

    30

    40

    50

    2

    53

    4

    8

    5

    2

    190

    200210

    220

    230

    240

    250

    4

    44

    2

    2

    4

    4

    290

    300310

    320

    330

    340

    350

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    Basic Tools for Process Improvement

    Step 3 -Compute the range for the data set. Compute the range by su

    the smallest value in the data set from the largest value. The range repthe extent of the measurement scale covered by the data; it is always a number. The range for the data in Viewgraph 8 is 590 yards. This num

    obtained by subtracting -180 from +410. The mathematical operation bdown in Viewgraph 8 is:

    +410 - (-180) = 410 + 180 = 590

    Remember that when you subtract a negative (-) number from another nbecomes a positive number.

    Step 4 - Determine the number of intervals required . The number of

    influences the pattern, shape, or spread of your Histogram. Use the foll

    table (Viewgraph 9) to determine how many intervals (or bars on the bayou should use.

    If you have this Use this number many data points: of intervals:

    Less than 50 5 to 7

    50 to 99 6 to 10

    100 to 250 7 to 12

    More than 250 10 to 20

    For this example, 10 has been chosen as an appropriate number of inte

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    HISTOGRAM VIEWG

    Largest value = + 410 yards past target

    Smallest value = - 180 yards short of targ

    Range of values = 590 yards

    Step 3 - Compute the range for the data s

    How to Construct a Histogram

    Calculation: + 410 - (- 180) = 410 + 180 = 5

    IF YOU HAVE THIS

    MANY DATA POINTS

    USE THIS NUMBER

    OF INTERVALS:

    Less than 50

    50 to 99

    5 to 7 intervals

    6 to 10 intervals

    Step 4 - Determine the number of interva

    required

    How to Construct a Histogram

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    Basic Tools for Process Improvement

    Step 5 - Compute the interval wid th . To compute the interval width (Vie

    10), divide the range (590) by the number of intervals (10). When com

    interval width, you should round the data up to the next higher whole nu

    come up with values that are convenient to use. For example, if the ranis 17, and you have decided to use 9 intervals, then your interval width i

    You can round this up to 2.

    In this example, you divide 590 yards by 10 intervals, which gives an int

    width of 59. This means that the length of every interval is going to be 5

    To facilitate later calculations, it is best to round off the value representinwidth of the intervals. In this case, we will use 60, rather than 59, as the

    width.

    Step 6 - Determine the starting point for each interval. Use the smal

    point in your measurements as the starting point of the first interval. Th

    point for the second interval is the sum of the smallest data point and thwidth. For example, if the smallest data point is -180, and the interval w

    the starting point for the second interval is -120. Follow this procedure(Viewgraph 11) to determine all of the starting points (-180 + 60 = -120; = -60; etc.).

    Step 7 - Count the number of poin ts that fall within each in terval. T

    the data points that are equal to or greater than the starting value and

    the ending value (also illustrated in Viewgraph 11). For example, if the

    interval begins with -180 and ends with -120, all data points that are equgreater than -180, but still less than -120, will be counted in the first intein mind that EACH DATA POINT can appear in only one interval.

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    HISTOGRAM VIEWG

    59Interval

    Width

    Range

    Number ofIntervals

    =590

    10

    =

    Use 10 for the

    number of intervals

    Use 10 for the

    number of intervals

    Round u

    to 60

    =

    Step 5 - Compute the interval width

    How to Construct a Histogram

    INTERVAL

    NUMBER

    STARTING

    VALUE

    ENDING

    VALUE

    1

    2

    3

    4

    5

    6

    -180

    -120

    -060

    000

    060

    120

    INTERVAL

    WIDTH

    60

    60

    60

    60

    60

    60

    -120

    -060

    000

    060

    120

    180

    2 0

    NUMBER OF

    COUNTS

    3

    5

    13

    20

    22

    24

    Step 6 - Determine the starting point of each inte

    Step 7 - Count the number of points in each inte

    How to Construct a Histogram

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    Basic Tools for Process Improvement

    Step 8 - Plot the data. A more precise and refined picture comes into vie

    you plot your data (Viewgraph 12). You bring all of the previous steps towhen you construct the graph.

    ! The horizontal scale across the bottom of the graph contains the intewere calculated previously.

    ! The vertical scale contains the count or frequency of observations wof the intervals.

    ! A bar is drawn for the height of each interval. The bars look like colu

    ! The height is determined by the number of observations or percenta

    total observations for each of the intervals.

    ! The Histogram may not be perfectly symmetrical. Variations will occ

    yourself whether the picture is reasonable and logical, but be careful

    your preconceived ideas influence your decisions unfairly.

    Step 9 - Add the title and legend. A title and a legend provide the who,

    when, where, and why (also illustrated in Viewgraph 12) that are importaunderstanding and interpreting the data. This additional information docthe nature of the data, where it came from, and when it was collected. T

    may include such things as the sample size, the dates and times involve

    collected the data, and identifiable equipment or work groups. It is impoinclude any information that helps clarify what the data describes.

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    HISTOGRAM VIEWG

    USS CROMMELIN (FFG-37), PACIFIC MISSILE FIRING RANGE, 135 BL&P ROUNDS/MOUNT 31, 2LEGEND:

    Step 8 - Plot the data

    Step 9 - Add the title and legend

    How to Construct a Histogram

    0

    5

    10

    15

    20

    25

    -180 -120 -060 000 060 120 180 240 300 360 420

    TARGET

    YARDS LONGYARDS SHORT

    MISS DISTANCE FOR MK 75 GUN TEST FIRING

    HITS

    MISSESSHO

    T

    COUNT

    MISSES

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    How do we interpret a Histogram?

    A Histogram provides a visual representation so you can see where most o

    measurements are located and how spread out they are. Your Histogram mshow any of the following conditions (Viewgraph 13):

    ! Most of the data were on target, with very little variation from it, as inViewgraph 13A.

    ! Although some data were on target, many others were dispersed awthe target, as in Viewgraph 13B.

    ! Even when most of the data were close together, they were located target by a significant amount, as in Viewgraph 13C.

    ! The data were off target and widely dispersed, as in Viewgraph 13D

    This information helps you see how well the process performed and how cowas. You may be thinking, "So what? How will this help me do my job bett

    with the results of the process clearly depicted, we can find the answer to aquestion:

    Did the process produce goods and services which are within specificatio

    Looking at the Histogram, you can see, not only whether you were withinspecification limits, but also how close to the target you were (Viewgraph 14

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    HISTOGRAM VIEWG

    Target Target

    Target Tar

    Interpreting HistogramsLocation and Spread of Data

    A

    DC

    B

    Interpreting HistogramsIs Process Within Specif ication

    Limits?

    WITHIN LIMITS OUT OF SPEC

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    Basic Tools for Process Improvement

    Portraying your data in a Histogram enables you to check rapidly on the nuthe percentage, of defects produced during the time you collected data. Bu

    you know whether the process was stable (Viewgraph 15), you wont be apredict whether future products will be within specification limits or determin

    course of action to ensure that they are.

    A Histogram can show you whether or not your process is producing produ

    services that are within specification limits. To discover whether the processtable, and to predict whether it can continue to produce within spec limits,

    to use a Control Chart (see the Control Chart module). Only after you havediscovered whether your process is in or out of control can you determine aappropriate course of actionto eliminate special causes of variation, or tofundamental changes to your process.

    There are times when a Histogram may look unusual to you. It might have one peak, be discontinued, or be skewed, with one tail longer than the othe

    shown in Viewgraph 16. In these circumstances, the people involved in the

    should ask themselves whether it really is unusual. The Histogram may nosymmetrical, but you may find out that it should look the way it does. On t

    hand, the shape may show you that something is wrong, that data from sev

    sources were mixed, for example, or different measurement devices were uoperational definitions weren't applied. What is really important here is to a

    jumping to conclusions without properly examining the alternatives.

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    HISTOGRAM VIEWG

    Interpreting HistogramsProcess Variation

    Target

    Day 1

    Target

    Day 2

    Target

    Day 3 Day 4

    Target

    Skewed

    (not symmetrical)

    Discontinued

    Interpreting HistogramsCommon Histogram Shapes

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    Basic Tools for Process Improvement

    How can we practice what we've learned?

    Two exercises are provided that will take you through the nine steps for dev

    Histogram. On the four pages that follow the scenario for Exercise 1 you wset of blank worksheets (Viewgraphs 17 through 23) to use in working throuof the exercises in this module.

    You will find a set of answer keys for Exercise 1 after the blank worksheetsExercise 2 after the description of its scenario. These answer keys represe

    one possible set of answers. It's all right for you to choose an interval widthnumber of intervals that is different from those used in the answer keys. Evthough the shape of your Histogram may vary somewhat from the answer k

    shape, it should be reasonably close unless you used a very different numbintervals.

    EXERCISE 1: The source of data for the first exercise is the following sce

    list of the data collected follows this description. Use the blank worksheetsViewgraphs 17 through 23 to do this exercise. You will find answer keys inViewgraphs 24 through 30.

    Your corpsman is responsible for the semiannual PhysicalReadiness Test (PRT) screening forpercent body fat. Prior

    to one PRT, the corpsman recorded the percent of body fatfor the 80 personnel assigned to the command. These are

    the data collected:

    PERCENT BODY FAT RECORDED

    11 22 15 7 13 20 25 12 16

    4 14 11 16 18 32 10 16 17

    8 11 23 14 16 10 5 21 26

    23 12 10 16 17 24 11 20 9

    24 10 16 18 22 15 13 19 15

    11 20 15 13 9 18 22 16 18

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    HISTOGRAM VIEWG

    Step 1 - Count the number of data pointWORKSHEET

    TOTAL NUMBER =

    WORKSHEET

    Step 2 - Summarize the data on a tally she

    VALUE TALLY VALUE TALLY VALUE TALLY VALUE TALLY VALUE T

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    HISTOGRAM VIEWGR

    Largest value = _______________

    Smallest value = _______________

    ________________________________________

    Range of values = _______________

    Step 3 - Compute the range for the data se

    WORKSHEET

    Step 4 - Determine the number of intervals

    WORKSHEET

    IF YOU HAVE THIS

    MANY DATA POINTS

    USE THIS NUMBER

    OF INTERVALS:

    Less than 50

    50 to 99

    5 to 7 intervals

    6 to 10 intervals

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    HISTOGRAM VIEWG

    Step 5 - Compute the interval width

    WORKSHEET

    Interval

    Width

    Range

    Number ofIntervals

    = =

    Round up

    next high

    whole num

    =

    Step 6 - Determine the starting point of each inteWORKSHEET

    INTERVAL STARTING INTERVAL ENDING NUMBER

    NUMBER VALUE WIDTH VALUE OF COUNTS

    1

    2

    3

    4

    5

    Step 7 - Count the number of points in each in ter

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    HISTOGRAM VIEWGR

    Step 8 - Plot the data

    Step 9 - Add ti tle and legend

    WORKSHEET

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    HISTOGRAM VIEWG

    Step 1 - Count the number of data points

    EXERCISE 1 ANSWER KEY

    11 22 15 7 13 20 25 12 16 1

    4 14 11 16 18 32 10 16 17 1

    8 11 23 14 16 10 5 21 26 1

    23 12 10 16 17 24 11 20 9 1

    24 10 16 18 22 15 13 19 15 2

    11 20 15 13 9 18 22 16 18

    14 20 11 19 10 17 15 12 17 1

    17 11 15 11 15 16 12 28 14 1

    TOTAL = 80

    Step 2 - Summarize the data on a tally she

    EXERCISE 1 ANSWER KEY

    0 0

    1 02 0

    3 0

    4 1

    5 1

    6 0

    %FAT NO. OF PERS

    11 9

    12 413 5

    14 4

    15 7

    16 8

    17 5

    %FAT NO. OF PERS

    22 3

    23 224 3

    25 1

    26 1

    27 0

    28 1

    %FAT NO. OF P

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    HISTOGRAM VIEWGR

    Step 3 - Compute the range for the data set

    EXERCISE 1 ANSWER KEY

    Largest value = 32 Percent body fat

    Smallest value = 4 Percent body fat

    _________________________________________

    Range of values = 28 Percent body fat

    Step 4 - Determine the number of intervals

    EXERCISE 1 ANSWER KEY

    Less than 50

    50 to 99

    5 to 7 intervals

    6 to 10 intervals

    IF YOU HAVE THIS

    MANY DATA POINTS

    USE THIS NUMBER

    OF INTERVALS:

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    EXERCISE 1 ANSWER KEYStep 6 - Determine the starting point of each inter

    Step 7 - Count the number of points in each interv

    INTERVAL STARTING INTERVAL ENDING NUMBER

    NUMBER VALUE WIDTH VALUE OF COUNTS

    1 4 + 4 8 3

    2 8 + 4 12 20

    3 12 + 4 16 20

    4 16 + 4 20 20

    5 20 + 4 24 10

    HISTOGRAM VIEWG

    3.5

    Step 5 - Compute the interval width

    EXERCISE 1 ANSWER KEY

    Interval

    Width

    Range

    Number ofIntervals

    = =

    Round u

    to 4

    =

    28

    8

    Use 8 for the number

    of intervals

    Basic Tools for Process Improvement

    B i T l f P I t

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    HISTOGRAM VIEWGR

    EXERCISE 1 ANSWER KEY

    LEGEND: USS LEADER (MSO-490), 25 JUNE 94, ALL 80 PERSONNEL SAMPLED

    4 8 12 16 20 24 28 32 360

    JUNE 94 PRT PERCENT BODY FATSATISFACTORY % BODY FAT

    PERCENT BODY FAT

    NO.OFPERSONNEL

    0

    4

    6

    8

    10

    12

    14

    16

    18

    20

    2

    Step 8 - Plot the data

    Step 9 - Add ti tle and legend

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    EXERCISE 2: The source of data for the second exercise is the followin

    A listing of the data collected follows this description. Use the blank work

    Viewgraphs 17 through 23 to do this exercise. You will find answer keys Viewgraphs 31 through 37.

    A Marine Corps small arms instructor was performing ananalysis of 9 mm pistol marksmanship scores to improve

    training methods. For every class of 25, the instructorrecorded the scores for each student who occupied thefirst four firing positions at the small arms range. The

    instructor then averaged the scores for each class,maintaining a database on 105 classes. These are thedata collected:

    AVERAGE SMALL ARMS SCORES

    160 190 155 300 280 185 250 285 200

    175 190 210 225 275 240 170 185 215

    270 265 255 235 170 175 185 195 200

    180 245 270 200 200 220 265 270 250

    255 180 260 240 245 170 205 260 215

    255 245 210 225 225 235 230 230 195

    230 255 235 195 220 210 235 240 200

    195 235 230 215 225 235 225 200 245

    220 215 225 250 220 245 195 235 225

    210 240 215 230 220 225 200 235 215

    220 230 225 215 225

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    HISTOGRAM VIEWGR

    160 190 155 300 280 185 250 285 200 16

    175 190 210 225 275 240 170 185 215 22

    270 265 255 235 170 175 185 195 200 26

    180 245 270 200 200 220 265 270 250 23255 180 260 240 245 170 205 260 215 18

    255 245 210 225 225 235 230 230 195 22

    230 255 235 195 220 210 235 240 200 22

    195 235 230 215 225 235 225 200 245 23

    220 215 225 250 220 245 195 235 225 23

    210 240 215 230 220 225 200 235 215 24

    220 230 225 215 225

    Step 1 - Count the number of data points

    TOTAL = 105

    EXERCISE 2 ANSWER KEY

    155 1

    160 1

    165 1

    170 3

    175 2

    180 2

    205 1

    210 4

    215 7

    220 8

    225 11

    230 9

    255 4

    260 3

    265 2

    270 3

    275 1

    280 1

    Step 2 - Summarize the data on a tally sheetEXERCISE 2 ANSWER KEY

    SCORE TALLY SCORE TALLY SCORE TALLY

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    HISTOGRAM VIEWG

    Step 3 - Compute the range for the data se

    EXERCISE 2 ANSWER KEY

    Largest value = 300 Points

    Smallest value = 155 Points

    __________________________________

    Range of values = 145 Points

    Step 4 - Determine the number of interval

    EXERCISE 2 ANSWER KEY

    Less than 50

    50 to 99

    5 to 7 intervals

    6 to 10 intervals

    IF YOU HAVE THIS

    MANY DATA POINTS

    USE THIS NUMBER

    OF INTERVALS:

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    EXERCISE 2 ANSWER KEY

    Step 6 - Determine the starting point of each interv

    Step 7 - Count the number of points in each interv

    INTERVAL STARTING INTERVAL ENDING NUMBER

    NUMBER VALUE WIDTH VALUE OF COUNTS

    1 155 + 15 170 3

    2 170 + 15 185 7

    3 185 + 15 200 11

    4 200 + 15 215 12

    5 215 + 15 230 26

    HISTOGRAM VIEWGR

    14.5

    Step 5 - Compute the interval width

    EXERCISE 2 ANSWER KEY

    Interval

    Width

    Range

    Number ofIntervals

    = =

    Round up

    to 15

    =

    145

    10

    Use 10 for the number

    of intervals

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    HISTOGRAM VIEWG

    155 170 185 200 215 230 245 260 275 290 300

    0

    5

    10

    15

    20

    25

    30

    SCORES

    NO.OFPERSONN

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    p

    REFERENCES:

    1. Brassard, M. (1988). The Memory Jogger, A Pocket Guide of Tools forContinuous Improvement, pp. 36 - 43. Methuen, MA: GOAL/QPC.

    2. Department of the Navy (November 1992), Fundamentals of Total QuaLeadership (Instructor Guide), pp. 6-44 - 6-47. San Diego, CA: Navy PResearch and Development Center.

    3. Department of the Navy (September 1993). Systems Approach to ProcImprovement (Instructor Guide), pp. 10-17 - 10-38. San Diego, CA: OU

    Quality Leadership Office and Navy Personnel Research and DevelopmCenter.

    4. Naval Medical Quality Institute (Undated). Total Quality Leader's CoursGuide), pp. U-26 - U-28. Bethesda, MD.

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