[playful page] A toy, not a test. It runs in your browser; nothing you do here is sent anywhere, and the only thing kept is a "played" tick in your own browser so the toy shelf can show it.

TOY #5 OF 6 works with a mouse, a finger or a keyboard

hich line is longer, the top or the bottom? One always is, by somewhere between 2% and 20%. The lines start in different places so you cannot just compare their ends. Twenty-five goes; arrow keys work too (up for top, down for bottom).

This toy needs JavaScript. The notes below still say what it would have shown.

What it shows. Weber found that the smallest difference a person can notice is roughly a constant fraction of what is being compared, not a constant amount (Weber, 1834) [paraphrase, not a quotation]: a few millimetres matter on a short line and vanish on a long one. Fechner built psychophysics on that observation (Fechner, 1860). The fraction itself differs from one kind of judgement to another; Teghtsoonian compared it across many senses and related it to the range each one covers (Teghtsoonian, 1971) [paraphrase, not a quotation]. Your table is a crude psychometric function, and the 75% point a crude just-noticeable difference. What it cannot show. Five trials per level is far too few for a real threshold; the staircase does it more efficiently. At the smallest differences on the shortest lines the gap is about one screen pixel, so your display's sharpness matters as much as your eyes. Weber's rule also tends to bend at the very small and very large ends of a range.

Sources

1. Weber (1834). De pulsu, resorptione, auditu et tactu: annotationes anatomicae et physiologicae. Leipzig: Koehler.
2. Fechner (1860). Elemente der Psychophysik. Leipzig: Breitkopf und Härtel.
3. Teghtsoonian (1971). On the exponents in Stevens' law and the constant in Ekman's law. Psychological Review. doi:10.1037/h0030300

see also

	Weber — touch, and the constant fraction
	Fechner — who turned it into a science
	the staircase — the proper way to find a threshold
	where the line is — an essay about thresholds