Table 1.

Considered factors related to the study methodology, population and intervention used to create the variables used in the analysis.

Study Methodology	Population	Intervention
- Use of intention-to-treat analysis	- Age	- Use of tDCS with a combined intervention
- Size of intervention groups	- Duration of symptoms	- Type of combined intervention
- Dropouts in intervention groups	- Gender	- Timing of combined intervention
- Disease diagnostic definition criteria	- Race	- Total number of intervention sessions
- Blinding of study subject	- Body mass index	- Frequency of intervention sessions per week
- Blinding of intervention staff	- Number of subjects with another chronic illness	- Duration of tDCS intervention in weeks
	- Baseline depression level	- Patient setting
	- Baseline anxiety level	- Type of tDCS montage
	- Baseline pain catastrophizing score	- Number of stimulation channels
	- Percentage of subjects using opioids	- Stimulation polarity
	- Percentage of subjects using gabapentinoids	- Active electrode location
	- Percentage of subjects using antidepressants	- tDCS current intensity parameter (mA)
	- Percentage of subjects using anticonvulsants	- tDCS session duration
	- Percentage of subjects using analgesics	- tDCS electrode surface area
	- Percentage of subjects using myorelaxants
	- Disability score at baseline
	- Sleep quality score at baseline
	- Baseline pain Level (Active and Sham groups)