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Practical guide · Root cause analysis and problem solving

Fishbone diagram (Ishikawa): how to draw one, read it and turn it into checks

Short answer

A fishbone diagram, also called an Ishikawa or cause-and-effect diagram, sorts the possible causes of one problem into categories such as the 6Ms, drawn as bones leading to the problem at the head. It is a brainstorming map, not proof: each cause is a suspect until checked.

A fishbone diagram puts one problem statement at the head of a spine and draws a bone for each category of cause, usually the 6Ms: Man, Machine, Method, Material, Measurement and Mother Nature. The team adds possible causes to each bone, then picks the few most likely and checks them with data on the floor. The diagram only lists suspects. A cause is confirmed when a check shows the problem follows it, and the 5 Whys then goes deeper on the confirmed one.

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What a fishbone diagram is

A fishbone diagram is a picture of every area a problem could come from, drawn so a team looks at all of them before settling on the first explanation that sounds right. The problem sits in a box at the head on the right. A horizontal spine runs to it, and angled bones off the spine carry the categories. Small branches on each bone hold the specific causes the team thinks of. It is named for its shape. It is also called an Ishikawa diagram after Kaoru Ishikawa, the Japanese quality expert who popularised it, and a cause-and-effect diagram after what it shows.

It is one of the seven basic quality tools. It is a way to organise a group's knowledge, so it is only as good as the people in the room and the checks that follow it. For the methods around it, see problem solving methods and the wider root cause analysis guide.

Which categories to put on the bones

The category names are a prompt, not a rule. Pick the set that makes the team think of causes it would otherwise miss.

Common category sets
SetCategoriesUse it for
6MsMan, Machine, Method, Material, Measurement, Mother Nature (environment)Manufacturing defects and stops. The default. See the 6 Ms of production
5MsThe 6Ms without MeasurementA quick first pass. Measurement is the one most often missed, so add it back if the problem is intermittent
8PsProduct, Price, Place, Promotion, People, Process, Physical evidence, PerformanceService and marketing problems, not shop floor ones
Your ownNames taken from your process steps, such as Mixing, Filling, Capping, InspectionWhen the problem sits in one process and the Ms feel generic

How to make a fishbone diagram in six steps

  1. 1

    Write a specific problem statement

    What, where, how big and since when, in one line, with a number. 'Leak test failures on filler line 2 rose from 1% to 6% of bottles since Monday' beats 'quality problems'. A vague head gives a vague diagram. The 5W1H template helps if the team cannot agree on the statement.

  2. 2

    Pick the right people

    Four to eight, including the operator who runs the process, maintenance, quality and someone who does not work there every day. Operators know things no data system holds.

  3. 3

    Draw the spine and the category bones

    Draw it on a whiteboard, paper or the fishbone board in Lean Studio, or print the fishbone diagram template. Label each bone with its category.

  4. 4

    Brainstorm causes onto each bone

    One idea per branch, written as something that could be true ('seal temperature drifts') and not as a verdict. Go around the bones one at a time. Stop criticising ideas during this step.

  5. 5

    Go one level deeper where it helps

    For a promising branch, ask what could cause that, and add a sub-branch. Two levels is usually enough. The 5 Whys is the better tool for one chain of cause.

  6. 6

    Choose, check and cross out

    Vote for the three to five most likely causes, write the check that would confirm or clear each one, assign an owner and a date, and mark each branch confirmed or cleared when the result is in. A fishbone with no checks beside it has not finished.

Worked example: a leaking cap (illustrative)

This example is made up. A bottling line's leak test failures rose from 1% to 6% of bottles. The team writes the head as 'Leak test failures on filler line 2: 6% since Monday, was 1%' and builds the fishbone below.

The fishbone as a table: bones, causes and the check for each shortlisted cause (illustrative)
BonePossible causes namedShortlisted check
ManNew operator on capper; torque set by feelCompare failures by operator across the last three shifts
MachineCapping head 3 clutch worn; chuck dirtyCount failures by capping head for one shift
MethodTorque setting not written down; changeover checklist skipped a stepCompare the torque on the setup sheet with the setting on the machine
MaterialNew cap lot; liner thickness variesRun 100 caps from the old lot and 100 from the new on the same head
MeasurementLeak tester pressure drifts; threshold changedTest 20 known-good and 20 known-leaking bottles on the tester
Mother NatureCold morning stiffens linersCompare failure rate by hour against the room temperature log

The team runs the checks that cost least. The tester passes all 20 known-good and catches all 20 known-leaking bottles, so Measurement is cleared. Counting failures by capping head for a shift, 24 of the 30 failed bottles came from head 3, which is 80% of the failures from one of six heads. The old and new cap lots both run clean on a good head, so Material is cleared. The torque on head 3 reads lower than on the other five, which points at the worn clutch. The confirmed finding is that head 3 does not hold its torque. A 5 Whys on that finding then asks why the clutch was not caught before it wore, and ends at a missing inspection step in the changeover checklist.

Notice what the fishbone did. It put six areas on the table, so the team did not stop at the new operator, which was the first guess in the room. It was the checks, not the diagram, that found head 3.

Mistakes that make a fishbone useless

Common mistakes and what to do instead
MistakeWhat goes wrongDo this instead
A vague headCauses are guesses about different problemsA specific, measured statement agreed before drawing
Stopping at the diagramOpinions are treated as findingsWrite a check, an owner and a date for every shortlisted cause
Writing people as causes'Operator error' ends the search and lets the process offAsk what let the error happen: training, setup, a missing instruction
Only the manager's viewThe person who runs the process is not askedInclude the operator and maintenance
Every bone filled equallyA hundred causes and no priorityVote, shortlist and check; leave empty bones empty
Using it for a single clear chainFour bones are filled to look thoroughUse the 5 Whys when there is one chain of cause

Fishbone, 5 Whys or Pareto: which first

Use a Pareto chart first when you have several problems and need to pick one. Use the fishbone when one problem could come from several areas and the team has to widen its view. Use the 5 Whys when the cause looks like one chain, or to go deeper on a cause the fishbone has confirmed. A fishbone then a 5 Whys is a common pair. For bigger, cross-functional problems the output of the fishbone goes into an A3 or 8D report.

Make one now

Download the fishbone diagram template, which is a printable six-M sheet plus the board in Lean Studio, and the root cause analysis template to keep the checks and the confirmed cause next to it.

Definitions and notes

  • The fishbone, cause-and-effect or Ishikawa diagram is one of the seven basic quality tools, attributed to Kaoru Ishikawa (ASQ, "Fishbone diagram"). Sources differ on the exact date and setting of its first use.
  • The 6Ms are as on the 6 Ms of production guide. The 8Ps are the service-marketing set; they are named here for comparison only.
  • The leaking-cap example, its counts and the cap lots are made up for illustration. They are not from a real plant.

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This page is the short version. These guides cover each part in full.

All our articles on this subject are in the root cause analysis and problem solving topic.

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FAQ

Questions about fishbone diagram

  1. It is used to map the possible causes of one problem across several categories, so a team looks at every area before choosing where to check. It is a brainstorming and organising tool in root cause analysis. It does not prove a cause: each cause it lists still has to be checked with data.

  2. In manufacturing the usual set is the 6Ms: Man, Machine, Method, Material, Measurement and Mother Nature (the environment). Other sets, such as the 8Ps for services, exist. The names are prompts, so you can replace them with the steps of your own process if that makes the team think of more causes.

  3. Write a specific problem statement in a box at the head, draw a spine and a bone for each category, brainstorm causes onto the bones, go one level deeper on promising ones, then vote for the most likely and write a check, an owner and a date for each. Mark each cause confirmed or cleared when the check is done.

  4. A fishbone spreads out across many categories of cause, and the 5 Whys follows one chain of cause downward. Use the fishbone when the problem could come from several areas. Use the 5 Whys when the cause looks like a single chain, or to take one confirmed cause from the fishbone to its root.

  5. Yes. Fishbone diagram, Ishikawa diagram and cause-and-effect diagram are three names for the same tool. It is called a fishbone for its shape and an Ishikawa diagram after Kaoru Ishikawa, the Japanese quality expert who popularised it.

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