In studying timing deviations a first distinction should be made between non-intended motor noise and intended expressive timing or rubato. The first category deviates in the range of 10 to 100 ms; the latter can deviate up to 50% of the notated metrical duration in the score.
Expressive timing is continuously variable and reproducible (Shaffer, Clarke & Todd, 1985) and clearly related to structure (Clarke, 1988; Palmer, 1989).
It is important to note that there is interaction between timing and the other expressive parameters (like articulation, dynamics, intonation and timbre). For example, a note might be accented by playing it louder, a fraction earlier than expected or by lengthening its sounding duration. Which method of accentuation is used is difficult to perceive, even when the accentuation itself is obvious.
To refer to expressive timing, in computer music the term micro tempo is often used, comparable to the term local tempo used in the psychology of music (the tempo changes from event to event, expressed as a ratio of a performance time interval and a score time interval). For clarity, the term timing would be more appropriate here. It specifies the timing deviation on a note-to-note basis and is often referred to as the expressive timing profile (Clarke, 1985; Shaffer, 1981; Sloboda, 1983), timing pattern or rubato pattern (Palmer, 1989).
In these patterns, points are often connected, either stepwise with straight line segments or with a smooth interpolation, yielding a timing curve. Only the first representation maintains a proper relation with the time map in which points are connected with line segments. These continuous time maps are used by Jaffe (1985) and most people of the computer music community. Time maps can be superimposed, using one for each voice.
Time maps can also be constructed for uniformly spaced units in the score like bars or beats. The corresponding duration patterns form a true tempo pattern. The points in these patterns can be connected by line segments, yielding so called tempo curves. Some authors insist on stepwise tempo changes, like Mathews (Boulanger, 1990), in which they are linked to one level of the metrical structure.