Comparison of whole‐head and split‐head design for the clinical evaluation of anti‐dandruff shampoo efficacy

OBJECTIVE: Dandruff is a common scalp condition that can be improved by regular use of shampoos containing anti‐fungal actives. The efficacy of anti‐dandruff shampoos can be assessed by measuring scalp flaking, one of the important dandruff symptoms. A randomized, double‐blind trial is often used with one of two clinical designs: whole‐head parallel design and split‐head paired design. We aimed to explore the difference in product differentiation between these two designs using the same two test shampoos and the same scalp flaking assessment method (Total Weighted Head Score Adhered Flakes—TWHS AF). METHODS: A clinical study was conducted with a 2‐ to 3‐week wash‐out phase and a 4‐week test phase, consisting of 2 cells: 120 subjects with whole‐head parallel design, divided into 2 subgroups (1:1) using on‐site controlled washing method (either wash their own hair at a study site, under the instruction of a study supervisor or wash their own hair at home, as per instructions, but without supervision) and 35 subjects with split‐head paired design using salon‐staff washing method. Both cells employed hair washing at frequency of three times a week and TWHS AF measurement once a week from the baseline assessment. RESULTS: Both designs gave similar differences in TWHS AF between products: 5.6 units (95% CI: 4.1–7.0 units) in whole‐head design and 5.9 units (95% CI: 4.9–6.9 units) in split‐head design. CONCLUSION: Split‐head paired design shows a similar ability of detecting product difference as whole‐head parallel design, whereas it is a choice of more efficient and more cost‐effective, as only a quarter of the subjects are required to demonstrate the efficacy between anti‐dandruff shampoos.