How much do illusions fool you?
Drag until the two shafts are the same length.
You have seen this illusion before. You have never been told your number.
Measured, not self-reported.
Illusions do not fool everyone equally
Almost everyone has seen this figure before. What almost nobody has been told is that illusions of this kind fool different people by different amounts, and that the amount is a reasonably stable property of the person rather than of the day they are having.
Cretenoud and colleagues put 76 observers, aged 6 to 66, through several different presentations of the Müller-Lyer figure and of the Ponzo illusion. Within each illusion the agreement was strong: somebody strongly affected by one version of Müller-Lyer was strongly affected by the others too.
The same study found something more awkward for the question this page asks in its title. Between the two illusions, the correlation was weak — being easy to fool by Müller-Lyer said little about being easy to fool by Ponzo. So there is no single dial marked "how much do your eyes lie to you". Whatever this test measures about you, it measures about this illusion.
Why this test has no published average behind it
Most tests on this site carry a reference distribution taken from published research, labelled with its real source and kept separate from responses collected here. This one carries none, and that is a decision rather than an omission.
The Müller-Lyer literature is old and large, but its numbers are tied to the apparatus that produced them — a particular display, a particular viewing distance, particular fin angles. An error measured by dragging a slider on your own screen, at whatever size and distance you happen to be sitting, is not the same quantity. Plotting the two on one axis would look rigorous and would be a category error.
So this one is ranked against the people who did exactly what you are about to do, on the same figure, and against nothing else. It needs a larger crowd before it can say anything, which is the price of not borrowing a number that does not fit.