Moody's, S&P and Fitch: sovereign rating methodologies compared

Moody's, S&P and Fitch dominate sovereign rating coverage and are recognised by regulators globally. For most institutional mandates, at least two of the three ratings determine eligibility, capital treatment and index inclusion, so a split rating is not a curiosity, it can be the difference between investment grade and high yield.

Each agency publishes its methodology openly. Reading them side by side is the fastest way to understand why one agency rates a sovereign two notches higher than another, and which of the three is likely to move first when fundamentals shift.

S&P Global Ratings scores five factors on a 1 (strongest) to 6 (weakest) scale: institutional assessment, economic assessment, external assessment, fiscal assessment (split into flexibility/performance and debt burden) and monetary assessment. The five scores combine into an indicative rating, which the sovereign committee can shift by up to one or two notches based on qualitative overlays.

Institutional strength carries significant weight, S&P is more willing than its peers to cut a sovereign several notches on governance deterioration alone, and its framework is the most explicit about penalising politicised central banks and weak checks and balances.

Moody's scorecard has four factors: Economic Strength, Institutions and Governance Strength, Fiscal Strength, and Susceptibility to Event Risk. The first three combine into an 'economic resiliency' score, which is then adjusted for event risk, political, government liquidity, banking sector or external vulnerability shocks.

The event-risk overlay makes Moody's the most reactive of the three to specific triggers: a banking crisis, a liquidity squeeze, a sudden political rupture. It also explains why Moody's ratings sometimes look 'stickier' during slow-burn fiscal deterioration, the framework rewards resilience buffers even as debt drifts higher.

Fitch's Sovereign Rating Model (SRM) is the most explicitly quantitative of the three. It runs a regression benchmarking each sovereign against peers on 18 variables (GDP per capita, growth volatility, government debt, external liquidity, reserves, governance indicators) producing a model rating.

The rating committee then applies a Qualitative Overlay of up to three notches across four categories: structural features, macro performance, public finances and external finances. Fitch's model tends to anchor closer to peer medians, which makes Fitch the least likely of the three to be the outlier rating, but also the fastest to move when quantitative inputs shift.

The sharpest analytical difference is how much weight each agency gives to politics. S&P's institutional pillar can drive multi-notch action on political risk alone; Moody's captures politics primarily through the event-risk lens (does this event impair debt service?); Fitch encodes governance quantitatively through World Bank indicators, which move slowly.

In practice: expect S&P to move first on a democratic backsliding story, Moody's to move first on a fiscal or banking event, and Fitch to move once the quantitative deterioration is measurable in the data.

A one-notch split across the three agencies is common and rarely a trading signal on its own. A two-notch split usually points to a genuine disagreement worth understanding, is one agency behind the curve, or is one over-weighting a factor the market prices differently?

The most useful diagnostic is direction: if two agencies share a Negative outlook and one is Stable, the Stable rating is the one at risk of catching down. Split ratings straddling the investment-grade/high-yield line (BBB- versus BB+) matter disproportionately because index and mandate eligibility often hinge on the lower of the two.