Term Structure of Interest Rates › Limitations and Practical Considerations

Building and using yield curves in practice involves several challenges that the idealized treatment above does not capture.

Real markets have bid-ask spreads and transaction costs. Choosing bid, ask or mid-prices changes the calibrated curve and the interpretation of its marks. Multiple curves may distinguish discounting from reference-rate projection, or different currencies, indices and collateral terms. The distinction is not generally a separate curve for existing positions versus new trades.

Liquidity varies across the curve. On-the-run Treasury issues are generally more actively traded than comparable off-the-run issues, with exceptions such as cheapest-to-deliver bonds. Instrument selection depends on the intended benchmark: the Federal Reserve's fitted Treasury curve, for example, excludes on-the-run and first-off-the-run issues to reduce special liquidity effects. Simply selecting the most liquid quotes does not remove every curve-construction bias.

Different coupons can give same-maturity bonds different yields to maturity because their payments weight a nonflat spot curve differently. They need not do so: on a flat curve with matched credit and conventions, different coupons can have identical yields to maturity. A high coupon alone does not establish a price premium to par; compare the coupon with the relevant discount rates. Tax effects are jurisdiction- and instrument-specific, and original-issue or market discount can be treated as interest rather than preferentially taxed capital gains.

Interpolation changes between-node rates and sensitivities even when the fitted nodes agree. Piecewise-linear zero rates generally create jumps in instantaneous forwards where interval slopes change; these features alone do not establish arbitrage or discontinuous bond prices. Cubic splines can oscillate or overshoot when nodes are sparse. The choice depends on the required smoothness, locality and instrument fit.

A curve needs an explicit observation time and purpose. Live pricing may require intraday updates as relevant quotes move, while a daily snapshot is suitable for other workflows. Yesterday's curve remains a historical as-of mark, not a current executable quote. The ECB's euro-area curves, for example, use business-daily production. Data quality, outlier checks and missing or stale quotes matter at either frequency.