How does the line length test work?
A horizontal line is drawn at the top. Below it is your line, which you lengthen or shorten with a slider until you think the two match, then press Done. In half of the 8 rounds your line stands upright; in the other half it lies flat, lower down and shifted sideways so its ends never line up with the first line's. The slider has no numbers on it, so there is nothing to read off: only your eye.
How is your result calculated?
Each answer is compared with the true length, and the miss is turned into a percentage of that length: a line set 10 units too long on a 100-unit target is 10% off. Your result is the average of those misses over all 8 lines, ignoring whether you were long or short. Lengths are measured in the drawing's own units, so the score doesn't depend on the size of your screen.
You also see your tendency for upright and flat lines separately: the average miss with its direction, such as "6% short". That split is there because of a well-known illusion, below.
Why do upright lines look longer?
A vertical line tends to look longer than a horizontal line of exactly the same length. This is the horizontal–vertical illusion, described in the 1850s and still studied today. If it affects you, you will stop sliding an upright line before it is really long enough, because it already looks as long as the reference. So the illusion shows up in your result as upright lines set short, while your flat lines come out closer to even.
Other drawings can fool the eye about length too. In the Müller-Lyer illusion, published by Franz Müller-Lyer in 1889, two equal lines look different because one ends in arrowheads pointing in and the other in arrowheads pointing out. This test keeps its lines plain, so the only trick left is the direction.
How to judge lengths more accurately
- Use a reference you can carry. Compare both lines with the same thing, such as the width of the play area or a finger held up to the screen.
- Expect the upright line to fool you. If your upright lines keep coming out short, push them a little further than looks right.
- Look at the ends, not the middle. Judging where each line starts and stops is more reliable than judging its length as a whole.
What this test does not measure
It measures how closely you matched 8 lines by eye, on your screen, in one sitting. It isn't an eye test or a test of vision, and a high miss says nothing about your eyesight. For another test of judging by eye, try Guess the Angle, or draw something yourself in Draw a Perfect Circle.