experiences was: even though the data sources and the analysis goals at first blush seemed disparate, the analyses almost invariably converged or are expected to converge toward a sometimes rudimentary, sometimes elaborate, customized data analysis system adapted to a particular data set. The reason of course is that in the case of large data sets many people will have to work for an extended period of time with the same or similar data.

A thing is massive, if it is too heavy to be moved easily. We may call a data set massive, if its mere size causes aggravation. Of course, any such a characterization is subjective and depends on the task, one's skills, and on the available computing resources.

In my position paper (Huber 1994b), I had proposed a crude objective classification of data by size, from tiny (10² bytes), small (10⁴), medium (10⁶), large (10⁸), huge (10¹⁰) to monster (10¹²). The step size 100 is large enough to turn quantitative differences into qualitative ones: specific tasks begin to hurt at well defined steps of the ladder. Whether monster sets should be regarded as legitimate objects of data analysis is debatable (at first, I had deliberately omitted the "monster" category, then Ed Wegman added it under the name "ridiculous"). Ralph Kahn's description of the Earth Observing System however furnishes a good argument in favor of planning for data analysis (rather than mere data processing) of monster sets.

Data analysis goes beyond data processing and ranges from data analysis in the strict sense (non-automated, requiring human judgment based on information contained in the data, and therefore done in interactive mode, if feasible) to mere data processing (automated, not requiring such judgment). The boundary line is blurred, parts of the judgmental analysis may later be turned into unsupervised preparation of the data for analysis, that is, into data processing. For example, most of the tasks described by Bill Eddy in connection with fNMR imaging must be classified as data processing.

• With regard to visualization, one runs into problems just above medium sets.
• With regard to data analysis, a definite frontier of aggravation is located just above large sets, where interactive work breaks down, and where there are too many subsets to step through for exhaustive visualization.
• With regard to mere data processing, the frontiers of aggravation are less well defined, processing times in batch mode are much more elastic than in interactive mode. Some simple standard data base management tasks with computational complexity O(n) or O(n log(n)) remain feasible beyond terabyte monster sets, while others (e.g. clustering) blow up already near large sets.