Critical to quality characteristics is a set of attributes or part of a process that is critical to quality or, more precisely, has a direct and significant impact on its actual or perceived quality.

For this reason, during a process improvement, it’s essential to identify these characteristics because by improving these few factors, we can drastically increase the quality. Remember that the customers have in mind only their primary needs. Therefore, it’s our work to identify the driver (the critical to quality characteristics) and the requirements of this driver.

Example: A customers need a website to sponsor his activity. Making the proper analysis, we can find that the two drivers are the Usability of the site from the users and its maintainability. About Usability, maybe the essential requirement can be speed and an easy user interface. About the maintainability, the velocity of adjourning it, and low cost of this adjourning.

Critical to quality tree

The CTQ Tree is a tool to identify the CTQ characteristics. If we look at the example we identified:

  • Needs: What the customer wants;
  • Driver: The important characteristics that are critical for the quality;
  • Requirements: Performance requirement;

We can depict it as a tree called the critical-to-quality tree like in the image1:

CTQ Tree correct example
Image1: Critical To Quality Tree (CTQT)

In the image2, we can have a bad CTQ Tree, why? What are the requirements of speed? less than one day? Less than ten seconds? And Easy to modify? We need to give something obvious to interpreter.

CTQ Tree bad example
Image2 : Critical To Quality Tree (CTQT) – BAD EXAMPLE

In a real scenario, we can have more than one needs, all connected to one root: Customer satisfaction.

Also, we can note that specific requirements (the last level of the tree) are easier to measure and take under control.

In the Y=F(X) equation format, the critical to quality characteristics (the driver) is the Y. The requirements can be looked at as some other X variable that impacts it. So in the example of image1, you have y1, which is Usability. In addition, you have x1, that is, “Loading time,” and x2, that is, “Time to find information.” In a Lean Six Sigma project, you want to find how (the F function) x1 and x2 impact Y.

House of quality

The house of quality is a valuable tool that helps to collect the CTQ and analyze them.

It depicts:

  • The voice of the customer on the row;
  • For each VOC, you put importance on the customer. You can use a scale from 1 to 5;
  • On the top of each column, you put the requirement that can influence the VOC;
  • In the intersection between VOC and requirement, you value the relationship. You can use a scale like Weak = 1, Medium = five, and High = 10;
  • The roof also identifies the correlation between the requirements. You can value this relationship with highly positive, positive, negative, and highly negative;
  • The last row has the sum of the relationship for each requirement. This helps to identify most important requirements that influence the VOC.
  • The last column can have a value that say how the competitor peform in the respect of each VOC;

In the image3 you can look at an example of House of quality:

Image3 - Houser of quality
Image3 – Houser of quality


  • The weights are 1,2 and 4 respectively for the customer requirements (VOC), 1,2, and 3;
  • The relationship between the customer (VOC) and functional requirement are depicted like a full circle (high), empty circle (medium), and triangle (weak). For example, from Voc1 and func.requirement1, you have a weak relationship.
  • The relationship between functional requirement (the roof) is depicted like ++ highly positive, + positive, – negative, and — highly negative. In the example from Func. requirement1 and three, you have a negative relationship (in fact, the – is the only value at the same distance from 1 and 3);
  • In the last row, you can easily find that the Func. Requirement 1 is the most important because it has a total score of 60;• In the last two columns, you can look at how you are performing in respect of one competitor.

For the exam you need to remember that:

  • Critical to Quality (CTQ) characteristics help to express the Voice Of the Customer (VOC) in a concrete, measurable way;
  • The CTQ Tree helps to identify these characteristics and the requirements that impact it;
  • The house of quality helps to identify the CTQ characteristics, the requirements that impact it, and the relationship among them;
  • Remember that the CTQ characteristics are the Y of our equation. The requirement that impacts the Y is the X.


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