In a paper looking at hand movements and time taken to complete a task it appears that the ratio of movements over time is constant (i.e., movement and time are highly correlated) yet they say in the paper they can control independently for both variable - how is this possible?
There is a strong relationship between the time taken and number of hand movements made (Spearman coefficient 0.79, P <0.01). This has been demonstrated with ICSAD before. Therefore, why not just time the procedure with a stopwatch? This is answered when we apply partial correlation coefficient tests. When controlling for time, the number of movements made significantly compares with surgical experience and global score (correlation coefficient −0.44 and 0.56, respectively, P <0.01 for both). However, when controlling for movement, the time taken had no such relationship with experience and global rating (correlation coefficient −0.02, P = 0.9; 0.10, P = 0.8, respectively), suggesting that operative speed is secondary to economy of hand movement.
Datta V, Chang A, Mackay S, Darzi A. The relationship between motion analysis and surgical technical assessments. Am J Surg. 2002 Jul;184(1):70-3.