How to play
After a short countdown the dots show for 0.5, 1 or 2 seconds, then vanish. The count is random between 10 and 250, drawn so that small and large numbers come up about equally often. Dot sizes vary a little so you cannot simply judge by the inked area.
Scoring
Your error is the ratio between your guess and the answer, so guessing double and guessing half count as equally wrong. Each round scores 10 × e^(−2.5 × |ln(guess ÷ answer)|): 10% off earns about 7.9, 25% too many about 5.7, and double the answer about 1.8.
Your number sense
People see small numbers at a glance, but for larger sets we fall back on an approximate sense of quantity. Its errors tend to grow in proportion to the number, so seeing 20 as 25 is about the same slip as seeing 200 as 250. Many studies have also found that people often underestimate large numbers. The results screen shows whether you tend to guess high or low.