Draw a perfect circle: method

What it measures

What it is: a hand-control game; how closely one closed freehand loop follows a circle. What it isn't: not a clinical tremor or motor test, and drawing direction has no health meaning.

Procedure

One stroke around a centre dot with a finger, pen or mouse (left button). While drawing, the line is coloured by each point's distance from the current best-fitting circle and the roundness is shown live. When the pointer lifts, the fitted circle is shown dashed. The page records the points (CSS px, 0.1 px resolution) with times in ms, thins them evenly to at most 600 points (first and last kept) and sends them with the drawing-area size.

Parameters

Scoring

Centre by Kåsa least-squares fit (minimising Σ(x² + y² + Dx + Ey + F)²). R = mean distance of the points from that centre. Deviation = mean |r_i − R| / R. Roundness = 100 × (1 − deviation), at least 0, in percent to one decimal; higher is rounder. Direction: clockwise or counterclockwise from the sign of the summed angle. Also reported: radius in px and relative to the area, angle covered, drawing time, number of points. The best score per input kind is kept only in the browser; comparisons stay within one input kind.

Valid-run rules

A drawing gets no score when it has fewer than 24 points or no fit ('too short'), a radius under 15% of the shorter side ('too small'), more than 430° ('too many loops'), less than 320° or a closing gap over 0.35 R ('not closed'), or takes more than 30 s ('too slow'). Point times must not go backwards and the drawing area must be 50–10,000 px per side.

Differences from the classic task

No standard lab version exists. Kinematic studies of drawing (e.g. Lacquaniti et al., 1983) record trajectories with digitising tablets and analyse speed and curvature; clinical drawing tasks such as the Archimedes spiral use different shapes and scores. Here a single loop is scored only for shape.

Limitations

Points arrive at the device's sampling rate, so slower parts of the line contribute more points to the mean. Live feedback lets players correct while drawing. Screen size, finger occlusion and mouse versus finger change scores. Practice raises scores. Not a medical test.

Languages and review status

Language-independent stimuli, instructions in all site languages.