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True and false interindividual differences in the physiological response to an intervention

What is the topic of this review? In ‘personalized medicine’, various plots and analyses are purported to quantify individual differences in intervention response, identify responders/non‐responders and explore response moderators or mediators. What advances does it highlight? We highlight the impact of within‐subject random variation, which is inevitable even with ‘gold‐standard’ measurement tools/protocols and sometimes so substantial that it explains all apparent individual response differences. True individual response differences are quantified only by comparing the SDs of changes between intervention and comparator arms. When these SDs are similar, true individual response differences are clinically unimportant and further analysis unwarranted.