# Root cause analysis: 5 Whys, fishbone diagrams and Pareto charts Source: https://sarang.pro/root-cause-analysis-5-whys-fishbone-pareto/ Author: Sarang | Published: 25 October 2026 When something goes wrong in a business, the first explanation is rarely the right one. A late delivery gets blamed on the courier, a bad quarter on the weather, a complaint on one careless employee. Fix the first explanation and the problem usually comes back. Root cause analysis is the discipline of asking what made the problem possible, not just who was nearby when it happened. Three simple tools do most of the work: Pareto charts to decide where to look, fishbone diagrams to map possible causes, and the 5 Whys to follow one chain of cause and effect to its end. This guide covers where each came from, how to use it, and where it falls short. ## Pareto charts: decide where to look The [Pareto principle](https://en.wikipedia.org/wiki/Pareto_principle) comes from the Italian economist Vilfredo Pareto, who wrote in 1906 that about 80% of Italy's land was owned by about 20% of the population. Joseph Juran, an American quality engineer, came across Pareto's work in 1941 and applied the idea to quality: most problems come from a small share of causes. Juran later described it as "the vital few and the useful many". The 80:20 split is a rule of thumb, not a law. Real ratios vary, and 90:10 or 70:30 are common. The two numbers also do not need to add up to 100, because they measure different things. A [Pareto chart](https://en.wikipedia.org/wiki/Pareto_chart) turns the idea into a picture. It shows categories as bars in descending order, with a line showing the cumulative percentage. It is one of the seven basic tools of quality control. To build one: - Pick the problem and a measure, such as number of complaints or cost of returns. - Group the data into categories, such as complaint reasons. - Sort the categories from largest to smallest and plot them as bars. - Add a cumulative percentage line on a second axis. - Focus on the few bars that account for most of the total. ## Fishbone diagrams: map the possible causes The [fishbone or Ishikawa diagram](https://en.wikipedia.org/wiki/Ishikawa_diagram) is named after Kaoru Ishikawa, who popularised it in the 1960s and pioneered quality management processes at the Kawasaki shipyards. The problem sits at the head of the fish. Major cause categories form the bones, and specific causes hang off each bone. Common category sets give you a starting checklist so you do not fixate on one area: - The 5 Ms from manufacturing: manpower, machine, material, method, measurement. - The 8 Ps from marketing: product, price, place, promotion, people, process, physical evidence, performance. - Service sets that include surroundings, suppliers, systems and skills. To run a fishbone session, write a specific problem statement at the head, choose four to six categories that fit your business, and ask the group for possible causes under each. Then mark the causes you have evidence for and the ones you need to check. A fishbone is a map of hypotheses, not a list of findings. ## The 5 Whys: follow one chain to its end The [5 Whys](https://en.wikipedia.org/wiki/Five_whys) technique was developed by Sakichi Toyoda and used within Toyota as part of its problem-solving training. Taiichi Ohno, the architect of the Toyota Production System, called it "the basis of Toyota's scientific approach by repeating why five times". The method is simple. State the problem, ask why it happened, then ask why again of each answer until you reach a cause you can fix that would stop the problem recurring. Five is a guide, not a rule. Wikipedia's example starts with cross-threaded bolts and ends, five answers later, at a rusted shelf foot that made a parts bin fall over. Emmanuel Jallas, writing for the [Lean Enterprise Institute](https://www.lean.org/the-lean-post/articles/getting-the-most-out-of-the-5-whys/), calls the 5 Whys one of lean's best known and trickiest tools. His advice: - If you are going round in circles or landing on vague answers such as "we must raise awareness", start again. - Do not stop at a person. People rarely make mistakes on purpose, so ask what allowed the mistake. - Go to where the work happens and watch it, looking at materials, methods and machines as well as people. ## Using the three tools together The tools work best in sequence. Use a Pareto chart to choose which problem deserves attention. Use a fishbone to list possible causes of that problem across several categories. Use data to rule causes in or out, then use the 5 Whys on the strongest candidates to reach something you can change. ## An example (illustration only) An online clothing shop sees returns rise and wants to know why. Step one is a Pareto chart. Over three months, returns are grouped by reason. "Wrong size" accounts for about 60% of returns, "changed mind" 20%, "faulty" 10% and other reasons 10%. Sizing is the vital few. Step two is a fishbone. The team maps possible causes of wrong-size returns under product (inconsistent sizing between suppliers), method (size guide out of date), people (customers ordering two sizes on purpose), and measurement (no fit data captured at returns). Returns data shows two suppliers account for most sizing complaints. Step three is a 5 Whys on the supplier sizing cause: - Why are customers returning items for size? Items from two suppliers run small. - Why does that surprise customers? The size guide shows one chart for all brands. - Why is there one chart? The guide was written when the shop sold one brand. - Why was it never updated? Nobody owns the size guide. - Why does nobody own it? New supplier onboarding has no step for checking size data. The fix is a size check in supplier onboarding and brand-specific guides, not a warning to the customer service team. ## Limits and common mistakes Each tool has known weaknesses. Teruyuki Minoura, a former Toyota managing director of global purchasing, called the 5 Whys "too basic a tool to analyze root causes at the depth necessary to ensure an issue is fixed". Alan Card argued in *BMJ Quality & Safety* in 2017 that the arbitrary depth of the fifth why is unlikely to match the true root cause. Other criticisms are that investigators stop at symptoms, are limited by what they already know, and reach different answers on the same problem. - Blaming a person. "Operator error" is where analysis stops, not where it should end. - Following one chain only. Most problems have several contributing causes. A fishbone helps you see them. - Treating a fishbone as proof. Causes on the diagram are guesses until data supports them. - Counting the wrong thing on a Pareto chart. The most frequent problem may not be the most costly. Try charting by cost as well as count. - Stopping at an action nobody can take. A root cause should point to a change in a process, standard or system. ## When to use each tool ToolUse it whenAvoid relying on it when Pareto chartYou have data on many problems and need to prioritiseCategories overlap or data is too thin to sort Fishbone diagramYou need a broad view of possible causes with a groupYou need proof; it only lists hypotheses 5 WhysThe problem is fairly contained and the people who know the process are in the roomThe problem is complex, high risk or has many interacting causes For serious incidents, such as safety failures, treat these tools as a starting point and use a more formal investigation method alongside them.