The general form of the “parts per” notation is used in a variety of settings in science and continuous improvement. Let’s focus on the specific term, parts per million, as it applies to continuous and process improvement.
Overview: What is parts per million (PPM)?
While you can have parts per hundred, parts per thousand, parts per billion and so on, parts per million (PPM) is one of the most common uses for the concept. Since PPM is a ratio (or fraction) of quantity and quantity terms, the resulting value is a pure unitless measurement number.
In the context of Lean Six Sigma, PPM would represent the number of defective parts per one million parts produced. For example, if your process produced 1,000,000 units of product, and inspection determined 458 of them to be defective, your PPM would be 458/1,000,000, or 458 PPM. This can be converted to a percentage of 0.0458%.
An industry example of parts per million (PPM)
A company embarking on a new continuous improvement program wanted to use PPM to compare two invoicing departments creating two different types of invoices. The company’s Black Belt offered these suggestions to the finance manager.
They inspected 1,000 invoices from one department and 750 from the other. To calculate PPM, the finance manager counted the total number of mistakes that occurred in each department. The first department had a total of 151 entry errors, and the second, 120 entry errors.
The PPM for department one was 151/1000 x 1,000,000, or 151,000 PPM. The second department was 120/750 x 1,000,000, or 160,000 PPM. Both were essentially the same.
Frequently Asked Questions (FAQ) about parts per million (PPM)
1. When is the term PPM used?
PPM has commonly been used in science and chemistry, and more recently in quality and continuous improvement. In science, it might be the number of contaminants per unit volume of liquid. Any ratio of units can be converted to PPM by multiplying the ratio by 1,000,000.
2. Can you give some examples of PPM?
It could be one inch in 16 miles or 1 second in 12 days.
3. Why would we use PPM?
It is a simply calculated unitless measurement that can be easily used to compare things. For example, if one lake has 25 parts of contaminant per million gallons, and another has 225 parts of contaminant per million gallons, you would say the second lake is more polluted.