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A speech timing lag is the time difference between corresponding events in two speech recordings. Define ${\Delta }_i=t_{B,i}-t_{A,i}$ for landmark $i$. Positive values mean speaker B's event occurs later; negative values mean B leads. The landmark might be a specified syllable onset or phrase boundary, so its definition is part of the measurement.

In synchronous speech experiments, Fred Cummins recorded paired readings and compared corresponding events. Typical differences were about $40\mathrm{ms}$ on average, with larger mismatches at phrase beginnings.^[ref]

Different summaries answer different questions. For illustrative lags $10,20,-30\mathrm{ms}$, the arithmetic mean is zero, the mean absolute value is $20\mathrm{ms}$, and the root mean square isZero average leadership therefore does not imply close alignment at every event. The square gives larger mismatches more influence on the final statistic.

Recording delays also matter. If channel B adds a known $12\mathrm{ms}$ delay relative to channel A, then ${\Delta }_{\mathrm{recorded}}={\Delta }_{\mathrm{speech}}+12\mathrm{ms}$ under this sign convention. Subtracting that calibrated offset separates equipment delay from speaker timing. Consistent landmark annotation and synchronized recording are consequently necessary for interpreting the measured lag.

Synchronous speech is a task in which speakers try to produce the same speech together in time. In paired reading, each speaker can hear a live partner and adjust the ongoing utterance. This provides an experimental form of joint speech with a specified text and measurable timing.^[ref]

In a 2003 study, Fred Cummins recorded 27 pairs under different conditions. Corresponding points in the two speech channels were typically about $40\mathrm{ms}$ apart on average; larger differences occurred at phrase beginnings. The study compared practice and access to visual information and found reciprocal timing adjustment relative to individual readings.^[ref]

A constructed measurement example shows why the statistic matters. Let a speech timing lag be ${\Delta }_i=t_{B,i}-t_{A,i}$ at landmark $i$. Offsets of $-30,+30,-30,+30\mathrm{ms}$ have an arithmetic mean of zero, although no landmark is simultaneous. Their mean absolute value is $30\mathrm{ms}$. Signed lag measures which voice leads; absolute lag measures temporal mismatch. Keeping both prevents alternating leadership from being mistaken for perfect alignment.

The measured landmark and aggregation rule are therefore part of the result. Close agreement at phrase endings does not imply equally close agreement at every consonant or phrase beginning.