Find and analyze possible causes

Collect and analyze information to determine possible causes in order to select the real cause of the problem.

  • Most used tools: 

    -Ishikawa diagram (including NGT).


    -Scatter plot

    -Check sheets


Ishikawa diagram

  • Definition: Diagram that shows the possible sorted causes of a problem.
  • Process elements:
  1. - Methods
  2. - Men
  3. - Raw materials
  4. - Measurement
  5. - Environment
  6. - Machinery and equipments


Steps: Based on a process of “brainstorming”:

1.- Each member generates a single idea (cause) each round. All of the ideas are numbered and registered.

2.- Discard repeated ideas.

3.- Verify that the remaining ideas are related to the problem (discuss).

4.- Place the resulting ideas into the appropriate

category in the Ishikawa (cause-and-effect) diagram. 


Example 1) Hypothetical example involving scrap in a certain area:




  • Definition: Technique used to analyze information based on the classification of its origins.
  • Some examples of strata are machinery by model or type, workers (training, shift,gender), raw material (supplier, lot), and the operating conditions (temperature,humidity...).

Example 1) Suppose a certain factory produces the same product in three different areas. Each area has three identical machines. The following table shows information on the number of defects in the three areas:

After analyzing the data a new table is obtained based on the information by area.

They now decide to stratify the above information for area 3:


Scatter plot

  • Definition: A simple graph of two variables. You can visualize the type and the degree of relationshiop (correlation) between these two variables. 


Type of relationship

Degree of relationship

  • How to do it? We will use the Microsoft Excel to help in the process.

Example 1) The table below shows information about the hardness of a group of manifolds vs. their solidification time. Draw and interpret a scatter plot. 



Check sheets

  • Definition: Forms to collect, analyze and report information. Standardize and facilitate the collection, analysis and reporting of information. 
  • Types

1.- To visualize distributions

2.- To score the total number of defects by type

3.- To locate defects

4.- To stratify the number of defective units

5.- Check list