Why does a PPM figure need an interval?

Because it comes from a count, and counts vary. Twelve events could have been eight or seventeen, and the PPM figures span nearly two to one.

My rate went from 180 to 250 PPM. Did it worsen?

On these counts there is no evidence that it did. The previous figure sits inside this period's interval, so the two are consistent with an unchanged process.

How do I get a tighter estimate?

More events, which usually means aggregating periods. Relative precision scales with 1 over the square root of the count.

Does this count defects that escape inspection?

No. Undercure often passes a quick check, so the detected rate is a lower estimate of the true one.

UV Curing calculator

UV Cure Defect Rate Calculator: PPM With Its Uncertainty

Convert a cure defect count into PPM and see what the count can support. Enter the defects, parts cured, PPM target and previous period.

What this calculator does

Turn a cure defect count into PPM with its interval, so a target or period comparison is judged against noise.

Formula used

Cure defect PPM = defects ÷ total cured parts × 1,000,000
95% interval on the count = defects ± 1.96 × √defects
Lower bound = (defects − margin) ÷ total parts × 1,000,000
Upper bound = (defects + margin) ÷ total parts × 1,000,000
Change from previous period = this period − previous period

Inputs explained

Cure Defects This Period: Cure-attributable defects this period.
Total Cured Parts This Period: Parts through the cure station in the same period.
PPM Target: The rate the process is expected to hold.
Previous Period PPM: Last period's figure, to judge the change.

How to use the result

Best suited to reporting a cure defect rate in a review, deciding whether a period-on-period change is real, testing whether a PPM target is observable.
Assumes a constant rate; real cure defects cluster when a lamp drifts. Assumes all cure defects were detected, which undercure often escapes. Says nothing about severity; catastrophic and cosmetic count the same.

Common questions

Why does a PPM figure need an interval? Because it comes from a count, and counts vary. Twelve events could have been eight or seventeen, and the PPM figures span nearly two to one.
My rate went from 180 to 250 PPM. Did it worsen? On these counts there is no evidence that it did. The previous figure sits inside this period's interval, so the two are consistent with an unchanged process.
How do I get a tighter estimate? More events, which usually means aggregating periods. Relative precision scales with 1 over the square root of the count.
Does this count defects that escape inspection? No. Undercure often passes a quick check, so the detected rate is a lower estimate of the true one.

Related guides

Last reviewed 2026-10-01.