The first distinction may seem rather trivial but it must nonetheless be
mentioned, because it affects specific properties of the
collected NL and SL data. While text generally stays on the paper when it is
written down, speech is transient. It is the nature of the phonetic facts which
speakers create during speech acts that they disappear at the moment
they come into existence.
The first difference (which in the step from speaking to writing has helped our cultural development) explains why to collect SL data is less trivial than to produce NL data. The former must necessarily be recorded, for example on a tape or a disk, to make it accessible for future use.