Knowledge base

What are Spread sizes?

Understanding Spread Sizes in Processes

Spread sizes are used to provide useful information regarding the distribution of data points on a measurement scale, showing the density/variation of observations on a process. I will explain this with a simple example.

Importance of Spread Sizes in Processes

Consider you’re a project manager for a project with a fixed deadline, for simplicity, consider the deadline as the midpoint. Most deliveries are made near the deadline, but of course, there will always be exceptions – some products arrive at the earliest or latest.

Understanding Variation within Processes

Essentially, spread sizes indicate how far the observations are from its mean. The distance of each delivery to the midpoint varies, a phenomenon statisticians call spread variation within the process.

At a fundamental level, this variation is important in that it impacts predictability and control. If measures are far from the mean, indicating a larger spread, it is a signal of increasing process variation. On the other hand, minimizing this variation results in a more predictable and stable operation – a primary goal of Lean methodology.

Operational Stability and Lean Methodology

Operational stability, according to Lean principles, means that all process facets should have variations controlled. One should produce the same or near-same performance every day, with seldom instances of exceptional performance.

Control of Process Variations

Variation has been essential in data analysis for understanding process dynamics, but when is a process “under control”? A sufficient number of process outputs should be due to common variations and very few for special reasons brought by exceptions and abnormalities.

Measurement of Spread Size: Range and Standard Deviation

Normal variation is linked to a procedure, and special variation results due to an unmannerly occurrence. Both are responsible for increasing the spread of data, which can be measured in terms of range and standard deviation.

Essentially, spread size or variation is a measure that gives a person a sense of direction in how to navigate the process. If a person can understand and control this spread size, then observational ability will be developed and used extensively to achieve and maintain a stable state of operations.

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