Long-time case researcher Harry Wolcott wrote in his manual (1990).
The critical task in qualitative research is not to accumulate all the data you can, but to “can” (i.e. get rid of) most of the data you accumulate. This requires constant winnowing. The trick is to discover essences and then to reveal those essences with sufficient context, yet not become mired trying to include everything that might possibly be described. Audiotapes, videotapes, and now computer capabilities entreat us to do just the opposite; they have gargantuan appetites and stomachs. Because we can accommodate ever-increasing quantities of data – mountains of it – we have to be careful not to get buried by avalanches of our own making.
Stake, R. 1995. The Art of Case Study Research. London: Sage. (p 84)
Wolcott, H. 1990. Writing Up Qualitative Research. Newbury Park, CA: Sage.