Ascientific investigation in which an individual's response to the treatment can be compared with the individual's own control value is called a study.

crossover

Explanation:

The cross-clinical trial (cross-over, for those who know English), in which each subject is their own control. As you can see in the attached figure, in this type of essay each subject is randomized to a group, they are subjected to the intervention, a washing or bleaching period is passed and they are subjected to the other intervention. Although this solution is not as elegant as that of the time machine, the defenders of cross-trials are based on the fact that the variability within each individual is less than the inter-individual, so that the estimate may be more accurate than that of the trial in parallel and, in general, smaller sample sizes are needed. Of course, before using this design you have to make a series of considerations. Logically, the effect of the first intervention should not produce irreversible changes or be very prolonged, because it would affect the effect of the second. In addition, the washing period has to be long enough to avoid any residual effect of the first intervention.

It is also necessary to consider whether the order of the interventions can affect the final result (sequence effect), so that only the results of the first intervention would be valid. Another problem is that, having a longer duration, the patient's characteristics may change throughout the study and be different in the two periods (period effect). And finally, be aware of the losses during the study, which are more frequent in longer studies and have a greater impact on the final results in the cross-tests than in the parallel trials.