A method of measuring spontaneous movements by time-sampling motion pictures.
1. A method is described of measuring spontaneous movements by time-sampling films.

2. This method is to be preferred to direct observation if frequently occurring movements are being studied because it is more accurate, less fatiguing, requires shorter periods of observation, and leaves the experimenter free. By using slow-motion, time-samples in units of 1 second are possible.

3. The split-half reliability of the movement scores obtained from the films was 0·93.

4. In two experiments it was found that an economical amount of film afforded a reliable sample of the subjects' movements. In the first, the autistic movements scores obtained from time-sampling 100 ft.of film correlated highly with the scores obtained from counting them with an E.M.G. during 20 minutes (r=0·83).
 In the second, tics were scored from short films and compared with the scores found during 12–30 minutes of direct observation. The results corresponded closely.

5. The validity of the method was tested: ( a ) by correlating the time-sampled scores of autistic movements obtained from films and from observation during identical periods (r=0·998); ( b ) by similarly correlating the film scores and E.M.G. scores of the movements obtained concurrently (r=0·87).

6. Although the numbers investigated were small the results suggest that the method is a reliable and valid one.