The z-score tells us how unusual something is. If a z-score is positive, then the larger it is the more unusually large the outcome is. Suppose that your friend's dog weighs 37 pounds and your friend's cat weighs 9.9 pounds. The dog's z-score in relation to all dogs is $\displaystyle \frac{{17}}{{13}}$, and the cat's z-score in relation to all cats is $\displaystyle \frac{{1.1}}{{0.4}}$. Is the dog or the cat more unusually large?