An illustration of possible data distributions in a task of accurate total force production while pressing with two fingers: F1 + F2 = C. For any value of C, there are an infinite number of solutions corresponding to a line with a negative slope on the force-force plane (dashed lines, UCM1, UCM2, and UCM3). If the person is asked to perform this task many times, the cloud of data points on the plane is expected to form an ellipse elongated along the dashed line and variance along that line is expected to be higher than in the orthogonal direction.