Subtraction method

Comparing reaction times for tasks that differ by one added step, and taking the difference as the time that step takes in the mind. The Dutch physiologist Franciscus Donders published it in 1868.

In an 1868 paper Donders reported measurements of the time taken by thinking itself^[1]. The decisive experiment was run on 21 August 1865: one person spoke a syllable and another repeated it as fast as possible. In the a-method the syllable was always 'ki'; in the c-method it was one of five and the listener answered only to 'ki'; in the b-method it was any of the five and he repeated what he heard. With Donders answering, the a-method took 201 milliseconds, the c-method 237 and the b-method 284^[2].

Donders read the c-minus-a difference, 36 milliseconds, as the time to tell which sound it was, and the b-minus-c difference, 47 milliseconds, as the time to choose which sound to say^[2].

That reading rests on an assumption about what happens inside the mind. In 1874 Wundt objected that the c-method also involves a choice, whether to answer or stay silent. In 2018 Ardi Roelofs went back to Donders's notebook and found that for two of his students, measured the same day, the c- and b-methods took almost the same time^[2].