= Synchronous speech
<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.<Practice and performance in speech produced synchronously by Fred Cummins>[Introduction and experimental task]{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.<Practice and performance in speech produced synchronously by Fred Cummins>[Abstract, Methods and synchrony results]{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.
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