Butzon 1997.

Methods	Quasi‐randomised cross‐over study
Participants	24 presbyopic volunteers from research and development personnel of a telecommunications equipment company who were age 47 or older wearing a full‐time multifocal lens design, needing a near add power of at least S+1.50 dioptres, that worked at a computer for 4 hours per day or more. Mean age: 53 years, range 47 to 66 Male/Female: 19/5 Mean time spent at a VDT per day: at least 4 hours (type of computer not described) Refractive error: not described, near add power of at least S+1.50 dioptres Country: USA
Interventions	Eyeglasses: progressive computer glasses (Technica by American Optical) vs. trifocal computer glasses (Datalite CRT trifocal by Vision‐Ease); each type of eyeglasses was worn for a period of 3 weeks, followed by a direct comparison during 3 weeks
Outcomes	Frequency and severity of symptoms at baseline and after each intervention period (3 and 6 weeks). Wearing time of the eyeglasses expressed as % of the working day.
Notes	Authors did not respond to our request for information about the allocation method.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Intervention and comparison in "counterbalanced order".
Allocation concealment (selection bias)	Unclear risk	Intervention and comparison in a "counterbalanced order"; no attempt to concealment reported.
Blinding of participants and personnel (performance bias) All outcomes	High risk	Blinding not possible because of differences in visual aspect of the eyeglasses: progressive vs. trifocal.
Blinding of outcome assessment (detection bias) Primary outcomes	High risk	No blinding of outcome assessment, as participants are outcome assessors.
Blinding of outcome assessment (detection bias) Secondary outcomes	High risk	No blinding of outcome assessment, as participants are outcome assessors.
Incomplete outcome data (attrition bias) All outcomes	Low risk	24 participants who all completed the study and answered all questionnaires.
Selective reporting (reporting bias)	Unclear risk	No study protocol. All outcomes mentioned in the methods section are reported.
Other bias	High risk	No wash‐out period between interventions.