46. (cross-over or cross over or crossover).tw.
47. placebo$.tw.
48. sham.tw.
49. (assign$ or alternate or allocat$ or counterbalance$ or multiple baseline).tw.
50. controls.tw.
51. (treatment$ adj6 order).tw.
52. or/16-51
53. 15 and 52
54. limit 53 to humans