The cause-effect diagram is a way of organizing and representing the different theories proposed about the causes of a problem. It is also known as the Ishikawa diagram or fishbone diagram and is used in the Diagnosis and Solution phases of the cause.
How to interpret a cause-effect diagram:
The cause-effect diagram is a vehicle to order, in a very concentrated way, all the causes that can supposedly contribute to a certain effect. Therefore, it allows us to achieve a common knowledge of a complex problem, without ever being a substitute for data. It is important to be aware that cause-effect diagrams present and organize theories. Only when these theories are contrasted with data can we prove the causes of observable phenomena. Common mistakes are constructing the diagram before globally analyzing the symptoms, limiting the proposed theories by unintentionally masking the root cause, or making mistakes in both the causal relationship and the order of theories, assuming significant time expense.
Next we will see how the value of a quality characteristic depends on a combination of variables and factors that condition the production process (among other processes). Let's use the example of making mayonnaise to explain the Cause-Effect Diagrams:
The variability of quality characteristics is an observed effect that has multiple causes. When a problem occurs with the quality of the product, we must investigate to identify the causes of it. To make a Cause-Effect Diagram we follow these steps:
1. We decide which is going to be the quality characteristic that we are going to analyze. For example, in the case of mayonnaise it could be the weight of the filled jar, the density of the product, the percentage of oil, etc.
We draw a thick arrow that represents the process and to the right we write the quality characteristic:
2. We indicate the most important and general causal factors that can generate the fluctuation of the quality characteristic, drawing secondary arrows towards the main one. For example, Raw Materials, Equipment, Operators, Measurement Method, etc.:
3. We incorporate in each branch more detailed factors that can be considered causes of fluctuation. To do this, we can ask ourselves these questions:
to. Why is there fluctuation or dispersion in the quality characteristic values? Due to the fluctuation of Raw Materials. Raw Materials are listed as one of the main branches.
b. Which Raw Materials produce fluctuation or dispersion in the quality characteristic values? Oil, Eggs, salt, other seasonings. Oil is added as a minor branch of the main Raw Materials branch.
c. Why is there fluctuation or dispersion in the oil? Due to the fluctuation of the quantity added to the mixture. We add to Oil the smallest branch Quantity.
d. Why is there variation in the added amount of oil? Due to irregular operation of the balance. The Balance branch is registered.
and. Why does the balance work irregularly? Because it needs maintenance. In the Balance branch we place the Maintenance branch.
Thus we continue to expand the Cause-Effect Diagram until it contains all possible causes of dispersion.
4. Finally we verify that all the factors that may cause dispersion have been incorporated into the diagram. The Cause-Effect relationships must be clearly established and in that case, the diagram is finished.
A Cause-Effect diagram is in itself educational, it helps people to know in depth the process with which they work, clearly visualizing the relationships between the Effects and their Causes. It also serves to guide discussions, clearly exposing the origins of a quality problem. And it allows you to find more quickly the assignable causes when the process deviates from its normal operation.
