Youden plot
Plot z scores for two samples and spot systematic error. Free, no sign-up. Everything runs in your browser; nothing you enter is sent or stored.
Try a Youden plot
One line per laboratory: its z score for sample A, then for sample B. Points beyond ±4 are drawn at the edge.
- 9 inside the ±2 square
- 2 outside, near the diagonal: high (or low) in both samples, which points to a systematic error
- 1 outside, away from the diagonal: one sample only, or random error
- 8 of 12 points sit in the ++ or −− quadrants.
How it works
A Youden plot puts each laboratory's z score for sample A against its z score for sample B. Points along the diagonal (high in both, or low in both) point to a systematic error, such as a calibration bias. Points far from the diagonal point to a problem with one sample, or to random error.
The dashed square marks ±2 on both axes. Many points in the two diagonal corners across a round means differences between laboratories (or methods) dominate.
For learning and for checking your own arithmetic. Your scheme's report and rules are what count.