The basic completeness percentage you see on a dog's pedigree (e.g. "91.1% complete") treats all missing ancestors equally. It's the total number of known ancestors divided by the total possible — a straightforward average.
The problem: a pedigree could be 100% complete on one side and 20% on the other, averaging out to 60% complete. That sounds reasonable, but the COI is unreliable because you're missing almost the entire dam (or sire) line. Any common ancestors on that side simply won't be found, and the COI will be artificially low.
PCI solves this by using the harmonic mean instead of the arithmetic mean.
The harmonic mean is a type of average that heavily penalises imbalance. The formula is:
This has a useful property: if either side approaches zero, the PCI approaches zero too, regardless of how complete the other side is.
Sire line: 90% complete, Dam line: 85% complete
When sire and dam completeness are roughly balanced, the harmonic mean and arithmetic mean give almost the same result. The harmonic mean only bites when things are uneven.
Sire line: 100% complete, Dam line: 20% complete
The PCI of 33% is a much more honest assessment. If half the pedigree is mostly empty, you really don't know what the COI is.
PCI is calculated per generation, separately for the sire and dam lines:
For each generation (1–10), count how many ancestors are known versus how many are possible, separately for the sire line and the dam line.
Average those per-generation ratios across all generations for each line. This gives sire-line completeness and dam-line completeness.
Combine the two using the harmonic mean formula above.
The per-generation breakdown is shown on the Inbreeding Analysis page, with a visual bar and fraction (e.g. "23/32") for each generation on each side. This lets you see exactly where the gaps are.
The PCI display shows sire and dam lines separately so you can see exactly where the gaps are. This is actionable information:
PCI is the foundation that determines how seriously to take all the other metrics. Think of it as the confidence indicator for everything else on the Inbreeding Analysis page.
Dog → Genealogy → Inbreeding Analysis
PCI appears directly below the Wright's COI value, with the sire/dam split and per-generation bars.