Scatter Diagram: Identify Relationships Between Two Variables in Manufacturing

What is a Scatter Diagram?

A Scatter Diagram is a quality improvement tool used to study the relationship between two variables.

It displays data points on a graph to help engineers identify whether changes in one variable are associated with changes in another.

Simple Factory Example

A manufacturing engineer wants to understand whether machine temperature affects product rejection.

The engineer collects data from five production runs:

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• 40°C → 2 rejected products
• 45°C → 4 rejected products
• 50°C → 7 rejected products
• 55°C → 11 rejected products
• 60°C → 15 rejected products

When these values are plotted, the points show an upward pattern.

This suggests that higher machine temperatures may be associated with more rejected products.

Three Common Relationships

• Positive Correlation: Both variables tend to increase together.
• Negative Correlation: One variable tends to decrease as the other increases.
• No Clear Correlation: No obvious relationship appears between the variables.

Why does it matter?

• Helps investigate possible causes of defects.
• Supports data-based problem-solving.
• Reveals patterns that may not be obvious from raw data.
• Helps engineers decide what to investigate further.

Important Insight

Correlation does not prove causation.

Higher machine temperature may be associated with more defects, but other factors could also be responsible. Further investigation and testing are necessary.

Practical Insight

When a quality problem occurs, don’t rely only on assumptions.

Collect data on the suspected factors and use a scatter diagram to check whether a relationship exists.