Then a z-score tells us how many standard deviations our sample is from that normal or most frequent value. Therefore, the z-score can tell us if the economy grew a lot ( z-scores > 2) or if a group that is sick has high temperatures (again a z-score > 2). Then what does a negative z-score tell us? That the sample was below the normal or most frequent value! So during the Great Recession the economy shrank.