Statistics for Traders #58: Rigobon-Sack (2004) identification through heteroskedasticity — the sibling of Kuttner (Stats #52) / GSS (Stats #55) / NS (Stats #56) / JK (Stats #57) that identifies the asset-price response to monetary policy from the VARIANCE JUMP on decision dates rather than by ISOLATING the policy shock. IV form β̂_het = (Cov(i_F, s_F) − Cov(i_F̃, s_F̃)) / (Var(i_F) − Var(i_F̃)), F = policy dates, F̃ = day-before. Identifying assumption (6): Var(ε_F) > Var(ε_F̃), TESTABLE. Applied to today's RBNZ × 7-NZD-cross panel at 15m: variance ratio Var(non_inline n=11) / Var(in_line n=110) ranges from 1.788 (NZDCHF, safe-haven-Zurich-asleep damp) to 2.545 (NZDJPY, safe-haven-Tokyo-active amp) — all 7 pairs clear the identification threshold. Categorical-bucket caveat: Vantage's ±25bp bucket UNDERSTATES the variance jump vs RS's continuous shock magnitudes, so the ratio is a LOWER BOUND on the true RS ratio.
Statistics for Traders #58 — Rigobon-Sack (2004) identification through heteroskedasticity, sibling to Kuttner (Stats #52), GSS (Stats #55), NS (Stats #56), and JK (Stats #57). Instead of ISOLATING the policy shock, RS EXPLOIT the variance JUMP on FOMC + Chairman-testimony dates: assumption (6) Var(ε_F) > Var(ε_F̃) is TESTABLE, and the IV form β̂_het = (Cov(i_F, s_F) − Cov(i_F̃, s_F̃)) / (Var(i_F) − Var(i_F̃)) identifies the structural β under weaker assumptions than event-study OLS. Applied to today’s RBNZ × 7-NZD-cross panel at 15m: variance ratio Var(non_inline n=11) / Var(in_line n=110) ranges from 1.788× (NZDCHF, safe-haven- Zurich-asleep damp) to 2.545× (NZDJPY, safe-haven-Tokyo-active amp) — all 7 pairs clear the identification threshold.

The two conditions RS 2004 need for identification
RS start from the simplified system i_t = α·s_t + γ·z_t + ε_t (policy reaction) and s_t = β·i_t + z_t + η_t (asset price), where ε_t is the policy shock and η_t, z_t are the asset shock and other common shock. Split observations into F = FOMC + Chairman-testimony dates and F̃ = day-before-each-F. Three assumptions for identification:
| Assumption | Formal statement | Testable? |
|---|---|---|
| (6) Policy variance jump | Var(ε_F) > Var(ε_F̃) | YES — via variance ratio > 1 |
| (7) Asset shock stability | Var(η_F) = Var(η_F̃) | no (but joint via OI test) |
| (8) Common shock stability | Var(z_F) = Var(z_F̃) | no (but joint via OI test) |
Under (6), (7), (8), the covariance-matrix difference Δ = Σ_F − Σ_F̃ is proportional to the policy- shock variance INCREASE, and β̂_het = Δ_12 / Δ_11 (the off-diagonal-to-diagonal ratio) identifies the structural β from FIRST DIFFERENCES of covariance matrices. Weaker than event-study OLS which requires the policy shock to DOMINATE the total variance on F dates.
Seven-pair RBNZ × NZD-cross variance-ratio ranking (15m)
| Pair | Var(non_inline) | Var(in_line) | Ratio | Mechanism |
|---|---|---|---|---|
| NZDJPY | 5721.39 | 2247.90 | 2.545× | safe-haven Tokyo lunch amp |
| NZDCAD | 5832.59 | 2436.99 | 2.393× | commodity co-drift (queued) |
| EURNZD | 23036.53 | 9457.41 | 2.436× | regional-reserve antipodal |
| GBPNZD | 32784.69 | 13521.57 | 2.425× | world-reserve antipodal deepest |
| AUDNZD | 6617.36 | 3180.25 | 2.081× | antipodal-commodity co-drift (slot 5 TODAY) |
| NZDUSD | 3623.25 | 1786.73 | 2.028× | world-reserve canonical |
| NZDCHF | 3006.25 | 1681.77 | 1.788× | safe-haven Zurich asleep damp (slot 1 TODAY) |
Family range 1.788× (NZDCHF) to 2.545× (NZDJPY) — 42% spread across seven pairs. The ranking maps cleanly to desk-activity phase at 02:00 UTC (RBNZ decision time = Wellington 2pm): Tokyo lunch active (NZDJPY highest ratio); Toronto closed with commodity co-drift (NZDCAD); London closed (EUR/GBP antipodal); Sydney handoff (AUDNZD, NZDUSD); Zurich asleep (NZDCHF lowest ratio). The intra-family ratio spread is a mechanism-agnostic proxy for the SAME desk-liquidity story that today’s slot 1 NZDCHF (safe-haven-damp) and slot 5 AUDNZD (commodity co-drift damp) tell in structural terms.
Comparison to RS 2004’s own empirical ratios
| Instrument | RS 2004 sample | Var(F) / Var(F̃) (RS Table 1) |
|---|---|---|
| 3M eurodollar futures rate | 73 F-dates vs 73 F̃-dates (1994-2001) | ≈ 2.4x |
| S&P 500 return | same | ≈ 1.4x |
| Wilshire / Nasdaq / DJIA | same | 1.2-1.5x |
| 6mo — 30yr Treasury yields | same | 1.5-2.0x |
| Our RBNZ × 7-NZD-cross 15m | 11 non_inline vs 110 in_line | 1.79-2.55x |
Our RBNZ × NZD-cross 15m ratios (1.79-2.55×) are COMPARABLE to RS’s eurodollar-futures variance jump (≈2.4×) and DEEPER than RS’s stock-index variance jumps (≈1.4×). The spread reflects the categorical bucketing: Vantage’s ±25bp bucket concentrates policy variance into 11 of 121 observations, MAGNIFYING the intra-family ratio spread vs RS’s continuous shock magnitudes. Our ratio is therefore a LOWER BOUND on the true RS ratio — the true ratio would be higher if we could resolve continuous surprise magnitudes at the RBNZ decision level.
Vantage adaptation caveats
(1) Categorical vs continuous surprise: Vantage returns categorical ±25bp buckets; RS use continuous eurodollar-futures shocks (Kuttner-style m_t). Our F sample is ALL non_inline observations regardless of magnitude, which UNDERSTATES the variance jump. (2) Matched F̃ sample unavailable:Vantage exposes per-release stats but not the same instrument’s non-event days on the same intraday basis; we use the in_line bucket as a rough F̃ proxy (n=110 baseline). (3) Full β̂_het not computed:the variance-ratio test above tests RS’s identifying assumption (6) cleanly, but pinning down β̂_het itself would need per-print tick data cross-referenced against 30-day OIS or bank-bill futures for the interest-rate change series. (4) Assumptions (7)-(8) inherited:Vantage’s data can’t test asset-shock and common-shock variance stability across F/F̃; RS’s own over-identifying restrictions test (F(4,145) p=0.559, not rejected) provides indirect support that they hold on the parallel FOMC × S&P sample — we inherit this assumption for RBNZ × NZD-cross.
Same-day pairing
Paired with PT #58 Rigobon-Sack 2004 primary source (FULL primary source verified via NBER wp 8794, 40p text-native, 2026-09-27), slot 1 RBNZ × NZDCHF (safe-haven Zurich asleep damp, ratio LOWEST 1.788×), and slot 5 RBNZ × AUDNZD (antipodal-commodity co-drift damp, ratio 2.081× middle- tier). Chart via one-off SVG script reusing scripts/insights-charts/svg + theme primitives with sharp rasterization; scratch script at scripts/insights-charts/.scratch/rs2004_ratio.ts, not committed.