Layton 1991.

Methods	Placebo‐controlled trial, within participants Number of centres not stated Period of inclusion not stated
Participants	Inclusion criteria Bilateral symmetrical and therapy‐resistant palmoplantar pustulosis of at least 1 year's duration (palms, soles, or both) No response to topical treatment Exclusion criteria Not stated Baseline data: topical PUVA therapy (26 soles; 18 palms) vs placebo (26 soles; 18 palms) Mean age, male/female, and duration of disease: not stated Proportion of participants with psoriatic lesions elsewhere: not specified Withdrawal: no dropouts
Interventions	Intervention 1 A: 0.75% 8‐methoxypsoralen in hydrophilic water/oil emulsion and UVA phototherapy 3 times per week (N = 26 soles; 18 palms) Intervention 2 B: placebo (hydrophilic water/oil emulsion before UVA phototherapy, with a Perspex plate inserted between palm/sole and UVA light source) 3 times per week (N = 26 soles; 18 palms) Co‐interventions: none Duration of treatment: 8 weeks
Outcomes	No primary or secondary outcome pre‐specified Percentage of palms/soles involved in the disease Pustule count Grade calculated for PPP according to an erythema, scaling, and fissuring score, where 0 = normal or near normal skin with minimal scaling; 1 = erythema with scaling only; 2 = erythema, scaling, and shallow fissures; and 3 = erythema, scaling, and deep fissures Subjective analysis of symptoms by participants on a 10‐cm horizontal linear analogue scale
Notes	Funding: not reported
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "At the initial visit each side was randomly allocated active or placebo therapy" Comment: insufficient information about the sequence generation process to permit judgement
Allocation concealment (selection bias)	Unclear risk	Comment: no information on method to guarantee allocation concealment
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Quote: "To the placebo‐treated side, the emulsion base only was applied for 10 min prior to exposure to the light source. A Perspex plate was inserted between the palm/sole and the UVA light source at the placebo‐treated side. These procedures were carried out so that the patient was unaware which side received active treatment" Comment: patients were probably blinded; however no details regarding blinding of personnel, so hence the risk is unclear
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Quote: "To the placebo‐treated side, the emulsion base only was applied for 10 min prior to exposure to the light source. A Perspex plate was inserted between the palm/sole and the UVA light source at the placebo‐treated side" Comment: no details on how the assessor was blinded
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "All the patients had treatment three times a week over the course of 8 weeks and all completed the trial" Comment: no missing outcome data
Selective reporting (reporting bias)	Unclear risk	Comment: no protocol found to guarantee that all planned outcomes are presented in the results