If you’re just tuning in, this post is part of a series aimed to summarize Overall Equipment Effectiveness (OEE) in clear, unambiguous language. This series started a few days ago with comments about a feature story in Industry Week magazine about OEE.
We’ve been working our way across the formula for OEE:
OEE = Availability * Performance * Quality
In this post we’re finally ready to define Quality.
Quality
As with Availability and Performance, Quality is a ratio, expressed as a percentage.
When calculating Quality, you can choose between two methods of calculation.
For both methods, the number of good pieces should include only those pieces that went through the process without rework or adjusting. In other words, Quality should be a measure of First Pass Yield.
Method 1: Ratio of Good parts to Total Parts
The first method is simple:
Quality = Number of Good Pieces / Total Pieces Produced
We know the total number of pieces produced from the examples in the prior posts. We just need to count up the number of defective pieces and calculate the number of good pieces.
| Total pieces produced | 1187 |
| Number of defective pieces | 78 |
| Number of good pieces | 1109 |
Quality = 1109 Good Pieces / 1187 Total Pieces Produced = 0.934 or 93.4%
Method 2: Ratio of Time producing Good parts to Time producing All parts
This method is more complex to calculate by hand – a great reason to let the computer do the work for you! – but is more accurate when you run different parts with different Ideal Cycle Times during the same shift.
Quality = ∑ (Good piecesi * Ideal Cycle Timei) / ∑ (Total piecesi * Ideal Cycle Timei)
As mentioned above, we know the Ideal Cycle Time and the total number of pieces produced from the examples in the prior posts. We just need to count up the number of defective pieces and calculate the number of good pieces.
| Ideal Cycle Time | 0.25 minutes (15 seconds) per piece |
| Total pieces produced | 1187 |
| Number of defective pieces | 78 |
| Number of good pieces | 1109 |
We only have one type of part, so the calculation is not difficult:
Quality = (1109 Good pieces * 0.25 minutes/piece Ideal Cycle Time) / (1187 Total pieces * 0.25 minutes/piece Ideal Cycle Time) = 0.934 or 93.4%
Comparison of the two Methods
To really understand the difference between the two methods, you need an example where different parts are run during the Shift.
| Part A | |
| Ideal Cycle Time | 0.25 minutes (15 seconds) per piece |
| Total pieces produced | 800 |
| Number of defective pieces | 17 |
| Number of good pieces | 783 |
| Part B | |
| Ideal Cycle Time | 0.75 minutes (45 seconds) per piece |
| Total pieces produced | 300 |
| Number of defective pieces | 30 |
| Number of good pieces | 270 |
Using Method 1 (ratio of parts), we get:
Quality = (783 + 270) Good Pieces / (800 + 300) Total Pieces Produced = 0.957 or 95.7%
Using Method 2 (ratio of times), we get:
Quality = ( (Good piecesA * Ideal Cycle TimeA) + (Good piecesB * Ideal Cycle TimeB) ) / ( (Total piecesA * Ideal Cycle TimeA) + (Total piecesB * Ideal Cycle TimeB) )
Quality = ( (783 pieces * 0.25min/piece) + (270 pieces * 0.75min/piece)) / ((800 pieces * 0.25min/piece) + (300 pieces * 0.75min/piece) )
Quality = (195.75 + 202.5) / (200 + 225) = 0.937 or 93.7%
As you can see, greater differences in Ideal Cycle Times will have a greater impact on the difference between these two Quality calculations.
How do you calculate Quality at your facility?
Please comment, schedule a conversation, or call us at 800-958-2709.
