Sociolinguistics 1, Exercise 1: inter or intra?

Two speakers in the same town pronounce -ing differently:
  inter-speaker variation (different speakers choose differently).

One speaker says -in' at home and -ing at a job interview:
  intra-speaker variation (the same speaker chooses differently on different
  occasions). The second case is also an example of formality (register)
  conditioning variation, which later chapters take up.

Source: Wikipedia, Variation (linguistics).