Big data is not about randomly looking for a needle in a haystack.The starting point is a business question or a strategic issue which needs facts based support. This is not any different from previous analytical approaches or facts based decision making.
The size of the data set does not matter. The power of big data does not necessarily come from the fact that there is more data, but that there is more interesting data. Big data is about merging a variety of datastreams (regardless of their size and whether realtime or not) and translating this wealth of information into knowledge that can tranform business processes.