Concentration

A lens that only ever points one direction is still a blind spot. skope watches your own news diet and warns you when it collapses onto too few dimensions — even when every article is individually reachable. This is the concentration watcher.

Effective-N

skope measures attention spread with Effective-N, the Inverse Simpson index over how your radar's impact distributes across axes:

Effective-N = 1 / Σ(pᵢ²)

where pᵢ is the share of total impact landing on axis i over the recent window. It answers: "effectively how many axes is my attention spread across?" — discounting axes that barely register.

• Effective-N ≈ your axis count → balanced. Your attention tracks your whole situation.
• Effective-N ≈ 1 → concentrated. Almost everything reaching you is one story — your portfolio, or one employer, or one technology.

Five axes each at 20% gives Effective-N = 5. But 80% on one axis and 5% on four others gives Effective-N ≈ 1.5 — nominally five axes, effectively one and a half.

Why concentration is the risk

Reachability already keeps the irrelevant out. The remaining failure mode is the opposite one: everything that reaches you is true, relevant, and pointed the same way. That's an echo chamber built from real signal — the hardest kind to notice from the inside, because nothing in it is wrong. Effective-N makes the shape of your attention legible so the collapse is visible before it becomes a habit.

Reading it in the brief

get_brief returns a concentration block alongside the radar. A low Effective-N is Claude's cue to flag the imbalance — "four of your last five briefs were your portfolio; here's what you're not seeing" — and, in a later version, to actively recommend a diverging query set.

Deterministic, like the rest of the ledger

Effective-N is pure math over the stored impact distribution — no model, no judgement call. skope computes the number; Claude interprets what to do about it. Same division of labor as everywhere else in skope: the ledger is deterministic, the synthesis is Claude's.

See also: Reachability, the lens that decides what enters the radar in the first place.