So there is a multitude of values to be extracted to pick up a musician’s expression in performance. If the music is written down, some of it is readable by the sheet music. Some of it however is an individual expression of the musician. which is far more abstract in character and much more difficult to pick up because it is not possible to predefine it or calculate it. So we have to quantize expression somehow directly from the performance. Clemens Wöllner suggests in his opinion article to quantify artistic expression with averaging procedures.
A big point of the expression is to raise the attractiveness of the musical piece one is playing to a point to make it one’s own in the sense of the performance. Individuality is highly valued in the expression of a performer. Cognitive psychology studies teach us that average modalities in visual and auditory modalities are viewed as more attractive. Averaging procedures typically produce very smooth displays in pictures and sound. Listeners of performance typically expect more from a concert or a recording than an even performance. As said individuality is highly appreciated in music.
In classical genres, expression is often added by subtle timing perturbations and fluctuations in dynamic intensity, as unexpected delays or changes in intensity that are different from the typical expectations of the listener can cause surprise and other emotional reactions and thus help the individual performer’s musical expression. In earlier decades of the 20th century, for instance, musicians typically employed large rubati which are deviations in note length, most of the melody voice. It is not as common anymore, the changes of note length are far smaller today. Research along these lines has for a long time studied expressive timing deviations from a non-expressive metronomic version. These timing deviations constitute an individual expressive microstructure. As performers are not able to render a perfect mechanical, metronomically exact performance. To quantify those timing variations using a so-called deadpan rendition as average, can not be a valid indicator of individuality.
So musical performances can be averaged according to the main quantifiable dimensions of duration, dynamic intensity, and pitch. As for the average performance, it was suggested in seminal studies 1997 by Repp that the attractiveness is raised by not deviating from the average, expected performance, but it is also considered a dull performance if there is no individuality in it by straying from the average.
Averaged deviations from the notated pitch in equidistant temperament could be analyzed. The sharpening or flattening of tones may reveal certain expressive intentions of individual performers. Also, musicians are able to shape the timbre of certain instruments to some extent which adds to their expression.
(see.: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3685802/#!po=71.7391 30.12.2021, 20:12.)