English to French Marketing Business/Commerce (general)

This term appears in a text about sales forecasting and the trial and repeat purchases of new products.

"A few years later, Gerald Eskin (1971) developed a so-called ‘stretch function’ that allows us to gain insights into the repeat rate of various classes of repeaters (“depth-of-repeat”)"

Despite numerous researches about Gerald Eskin, I can't find a French equivalent for this "stretch function" and also for "depth-of-repeat"...

Thanks !!