Table 3.

Main sources of bias in the studies of tDCS in behavioral addictions.

SELECTION BIAS
Method of recruiting subjects (healthy participant, non-healthy participant, with or without treatment participants).
Duration and severity of the addiction or related disorder. Stage of treatment prior to tDCS (detoxification or continuation of substance use).
OBSERVATION BIAS
Over or underestimating the intensity of craving.
Placebo effect of tDCS itself.
Placebo effect of therapeutic trials carried out in the field of addiction and related disorders.
Order of the placebo session and active session in a crossover study.
Insufficient number of pulses and number of sessions.
Attrition bias (drop out).
CONFOUNDING BIAS
Sociodemographic characteristics: age, gender, ethnicity.
Hormonal status.
Volume of gray matter.
Psychiatric and somatic comorbidities.
Handedness.
Psychotropic treatments (in particular, continuation of anti-craving drugs during the trial).
Duration of the session, which may overlap with the duration required for the craving to subside naturally.
Cumulative and persistent effects of tDCS when the interval between two sessions is very short.
Sample size.
Ability of the treatment-seeking participants to use relapse prevention techniques during cue-induced craving procedure.

All these biases are discussed in Sections Methodological Issues and Technical Issues.