Table 1.
Characteristics of studies included in the classical conditioning with verbal suggestion (pain) meta-analysis.
| Author | Year | N | Sample age | Percent female | Sensation induction method | Placebo manipulation | Rating scale | Acquisition trials (Placebo/Control) | Evocation trials (Placebo/Control) | Calibrated stimulus intensity difference (0-100)* | First or mean outcome measure | Risk of bias score | Comparison | Outlier based on funnel plot |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Au Yeung7 | 2014 | 20 | 19.8 | 59% | Electrical | Sham TENS | 0-100 VAS | 32 (16P/16C) | 32 (16P/16C) | NA | First | 3 | W | |
| Barnes11 | 2021 | 62 | 19.4 | 52% | Electrical | Sham TENS | 0-10 pain intensity | 30 (15P/15C) | 20 (10P/10C) | 45 | Mean | 3 | W | |
| Case21 | 2019 | 28 | NR | 53% | Thermal | Inert gel | 0-80 pain intensity | 8 (4P/4C) | 16 (8P/8C) | NA | Mean | 7 | W | − |
| Choi23 | 2011 | 15 | 25.3 | 0% | Electrical | Sham IV | 0-100 NRS | Unknown | 10 (5P/5C) | NA | Mean | 3 | W | |
| Chouchou24 | 2015 | 26 | 23.4 | 46% | Thermal | Inert gel | 0-100 VAS | 16 (8P/8C) | 10 (5P/5C) | 35 | Mean | 4 | W | |
| Colagiuri27 | 2018 | 21 | 20.2 | 71% | Electrical | Sham TENS | 0-100 VAS | 32 (16P/16C) | 32 (16P/16C) | NA | First | 5 | W | |
| Colloca30 | 2006 | 10 | 22.7 | 83% | Electrical | Sham electrode | 0-10 NRS | 36 (18P/18C) | 12 (6P/6C) | NA | Mean | 5 | W | + |
| Colloca34 | 2008 | 15 | 22.5 | 100% | Electrical | Sham electrode | 0-10 VAS | 24 (12P/12C) | 12 (6P/6C) | NA | Mean | 3 | W | |
| Colloca35 | 2008a | 16 | 32.0 | 66% | Laser | Inert gel | 0-10 NRS | 30 (15P/15C) | 30 (15P/15C) | NA | Mean | 5 | W | |
| Colloca31 | 2009 | 16 | 22.6 | 100% | Electrical | Sham electrode | 0-10 NRS | 24 (12P/12C) | 12 (6P/6C) | NA | Mean | 5 | W | |
| Colloca33 | 2010 | 46 | 22.8 | 65% | Electrical | Sham electrode | 0-10 VAS | 20 (10P/10C) | 40 (20P/20C) | 30 | Mean | 3 | W | + |
| Colloca33 | 2010 | 46 | 22.8 | 65% | Electrical | Sham electrode | 0-10 VAS | 80 (40P/40C) | 40 (20P/20C) | 30 | Mean | 3 | W | + |
| Colloca36 | 2019 | 53 | 28.1 | 64% | Electrical | Sham electrode | 0-10 NRS | 18 (9P/9C) | 36 (18P/18C) | 60 | Mean | 3 | W | |
| Colloca29 | 2020 | 400 | 29.4 | 59% | Thermal | Sham electrode | 0-100 VAS | 24 (12P/12C) | 12 (6P/6C) | NA | Mean | 5 | W | |
| Corsi37 | 2017 | 46 | 27.4 | 52% | Thermal | Sham electrode | 0-100 VAS | 12 (6P/6C) | 6 (3P/3C) | NA | Mean | 3 | W | |
| de Jong39 | 1996 | 36 | 21.3 | 100% | Electrical | Inert gel | 0-100 VAS | 20 (10P/10C) | 10 (5P/5C) | 25 | Mean | 5 | B | |
| De Pascalis40 | 2002 | 36 | 25.4 | 65% | Electrical | Inert gel | 0-10 VAS | 12 (6P/6C) | 30 (15P/15C) | NA | Mean | 2 | W | |
| De Pascalis43 | 2021 | 56 | 23.3 | 100% | Cold cup | Inert gel | 0-100 NRS | 2 (1P/1C) | 2 (1P/1C) | NA | Mean | 4 | W | |
| Egorova48 | 2020 | 24 | NR | 50% | Thermal | Inert gel | 0-20 Gracely scale | 48 (24P/24C) | 24 (12P/12C) | 25 | Mean | 5 | W | |
| Eippert49 | 2009 | 19 | 25.0 | 0% | Thermal | Inert gel | 0-100 VAS | 12 (6P/6C) | 30 (15P/15C) | 40 | Mean | 6 | W | |
| Eippert50 | 2009a | 13 | 25.0 | 0% | Thermal | Inert gel | 0-100 VAS | 12 (6P/6C) | 30 (15P/15C) | 40 | Mean | 0 | W | |
| Feldhaus55 | 2021 | 624 | 24.6 | 60% | Thermal | Inert gel | 0-100 VAS | 16 (8P/8C) | 16 (8P/8C) | 40 | Mean | 3 | W | |
| Flaten57 | 2018 | 25 | 21.9 | 56% | Thermal | Inert pill | 0-10 NRS | 3 (2P/1C) | 2 (1P/1C) | NA | Mean | 0 | W | |
| Frangos58 | 2021 | 46 | 39.7 | 85% | Thermal | Inert gel | 0-200 VAS | 24 (12P/12C) | 20 (10P/10C) | 45 | Mean | 3 | W | |
| Freeman59 | 2015 | 24 | NR | 50% | Thermal | Inert gel | 0-20 Gracely scale | 18 (9P/9C) | Unknown | 25 | Mean | 5 | W | |
| Gaab60 | 2019 | 81 | 25.2 | 60% | Thermal | Inert gel | 0-10 VAS | 16 (8P/8C) | 4 (2P/2C) | 30 | Mean | 3 | W | |
| Geisler62 | 2020 | 33 | 27.4 | 0% | Thermal | Inert gel | 0-100 VAS | 16 (8P/8C) | 8 (4P/4C) | 40 | Mean | 3 | W | |
| Geuter63 | 2013 | 40 | 26.0 | 0% | Thermal | Inert gel | 0-100 VAS | 24 (12P/12C) | 30 (15P/15C) | 50 | Mean | 3 | W | |
| Geuter63 | 2013 | 40 | 26.0 | 0% | Thermal | Inert gel | 0-100 VAS | 24 (12P/12C) | 30 (15P/15C) | 30 | Mean | 3 | W | |
| Grahl66 | 2018 | 23 | 24.6 | 0% | Thermal | Sham TENS | 0-100 VAS | 24 (12P/12C) | 24 (12P/12C) | 40 | Mean | 0 | W | |
| Hartmann67 | 2021 | 45 | 23.8 | 51% | Electrical | Inert gel | 0-8 pain intensity | Unknown | 32 (16P/16C) | 30 | Mean | 1 | W | |
| Huneke74 | 2013 | 73 | 37.6 | 66% | Laser | Inert gel | 0-10 NRS | 20 (10P/10C) | 20 (10P/10C) | 40 | Mean | 3 | B | |
| Jarcho77 | 2016 | 15 | 24.3 | 100% | Thermal | Inert gel | 0-100 VAS | 2 (1P/1C) | 2 (1P/1C) | NA | Mean | 4 | W | |
| Kirsch79 | 2014 | 48 | 26.4 | 50% | Thermal | Sham acupuncture | 0-20 Gracely scale | Unknown | Unknown | 40 | Mean | 5 | B | |
| Klinger80 | 2007 | 12 | 26.1 | 50% | Electrical | Inert gel | 0-8 pain intensity | 10 (5P/5C) | 10 (5P/5C) | 25 | Mean | 8 | W | |
| Kong82 | 2006 | 16 | 28.4 | 44% | Thermal | Sham acupuncture | 0-20 Gracely scale | 48 (24P/24C) | 24 (12P/12C) | 40 | Mean | 5 | W | |
| Laverdure-Dupont85 | 2009 | 38 | 23.4 | 58% | Thermal | Inert gel | 0-100 VAS | 16 (8P/8C) | 10 (5P/5C) | 20 | Mean | 3 | W | |
| Lee86 | 2020 | 21 | 23.6 | 43% | Pressure | Inert gel | 0-100 VAS | 12 (6P/6C) | 12 (6P/6C) | NA | Mean | 5 | W | |
| Lui89 | 2010 | 31 | 23.5 | 58% | Laser | Sham electrode | 0-100 VAS | 24 (12P/12C) | 12 (6P/6C) | NA | Mean | 7 | W | |
| Martin95 | 2010 | 40 | 21.2 | 70% | Thermal | Inert gel | 0-10 NRS | 16 (8P/8C) | 2 (1P/1C) | 30 | Mean | 5 | W | |
| Martini96 | 2015 | 28 | 23.5 | 50% | Laser | Inert gel | 0-100 NRS | 24 (12P/12C) | 32 (16P/16C) | NA | Mean | 7 | W | |
| Martin-Pichora94 | 2011 | 15 | 22.8 | 68% | Thermal | Inert gel | 0-10 NRS | 16 (8P/8C) | 2 (1P/1C) | 30 | Mean | 3 | W | |
| Montgomery104 | 1997 | 24 | NR | 50% | Electrical | Inert gel | 0-10 VAS | 20 (10P/10C) | 12 (6P/6C) | 30 | Mean | 9 | B | |
| Morton106 | 2009 | 66 | 25.0 | 64% | Laser | Inert gel | 0-10 pain intensity | 60 (30P/30C) | 60 (30P/30C) | 40 | Mean | 5 | B | |
| Morton105 | 2010 | 56 | 25.0 | 62% | Laser | Inert gel | 0-10 pain intensity | 60 (30P/30C) | 60 (30P/30C) | 40 | Mean | 7 | B | |
| Power117 | 2020 | 57 | 49.0 | 65% | Laser | Inert gel | 0-10 NRS | 20 (10P/10C) | 20 (10P/10C) | 40 | Mean | 2 | W | |
| Price118 | 1999 | 34 | 19.3 | 60% | Thermal | Inert gel | 0-10 VAS | 30 (15P/15C) | 4 (2P/2C) | 40 | Mean | 6 | W | |
| Rhudy120 | 2018 | 33 | 36.4 | 51% | Electrical | Inert gel | 0-100 VAS | 24 (12P/12C) | 24 (12P/12C) | NA | Mean | 6 | W | + |
| Rosén124 | 2016 | 36 | 25.0 | 58% | Thermal | Sham electrode | 0-100 NRS | 3 (1P/2C) | 3 (1P/2C) | NA | Mean | 0 | W | |
| Rütgen126 | 2015 | 102 | 26.2 | 67% | Electrical | Inert pill | 0-7 pain intensity | 4 (2P/2C) | Unknown | 25 | Unknown | 3 | B | |
| Schafer127 | 2015 | 40 | NR | 67% | Thermal | Inert cream | 0-100 VAS | 16 (8P/8C) | 40 (24P/16C) | 40 | Mean | 6 | W | |
| Schafer127 | 2015 | 40 | NR | 67% | Thermal | Inert cream | 0-100 VAS | 112 (56P/56C) | 40 (24P/16C) | 40 | Mean | 6 | W | |
| Schenk129 | 2017 | 24 | 25.4 | 48% | Thermal | Sham TENS | 0-100 VAS | 18 (9P/9C) | 18 (9P/9C) | 40 | Mean | 3 | W | |
| Skvortsova131 | 2020 | 37 | 23.1 | 0% | Thermal | Inert nasal spray | 0-10 NRS | 24 (12P/12C) | 20 (10P/10C) | 30 | Mean | 0 | W | |
| Tang138 | 2019 | 30 | 22.5 | 67% | Electrical | Sham TENS | 0-100 graphic rating scale | 32 (16P/16C) | 4 (2P/2C) | NA | Unknown | 4 | W | |
| Tu139 | 2021 | 27 | 27.4 | 46% | Thermal | Inert cream | 0-20 Gracely scale | 48 (24P/24C) | 24 (12P/12C) | 25 | Mean | 3 | W | |
| Valentini141 | 2014 | 27 | 24.9 | 54% | Laser | Sham electrode | 0-100 VAS | 24 (12P/12C) | 8 (4P/4C) | 42 | Mean | 5 | W | |
| Vambheim142 | 2021 | 59 | 21.5 | 44% | Thermal | Inert gel | 0-10 NRS | 30 (15P/15C) | 30 (15P/15C) | NA | Mean | 0 | W | − |
| Vambheim142 | 2021 | 32 | NR | 54% | Electrical | Inert gel | 0-10 NRS | 22 (11P/11C) | 36 (18P/18C) | 20 | Mean | 0 | W | |
| Wager152 | 2004 | 24 | NR | NR | Thermal | Inert gel | 0-10 VAS | 60 (12P/12C) | 12 (6P/6C) | 60 | Mean | 12 | W | |
| Wager149 | 2006 | 39 | 23.2 | 55% | Laser | Inert gel | −2 to 10 VAS | 10 (5P/5C) | 80 (40P/40C) | NA | Mean | 7 | W | |
| Wager150 | 2007 | 15 | NR | 0% | Thermal | Inert gel | 0-10 VAS | 10 (5P/5C) | 60 (30P/30C) | NA | Mean | 7 | W | |
| Watson154 | 2006 | 24 | 23.8 | 55% | Laser | Inert gel | 0-100 NRS | 20 (10P/10C) | 20 (10P/10C) | NA | Mean | 7 | W | |
| Watson156 | 2007 | 18 | NR | 45% | Laser | Inert gel | 0-10 pain intensity | 80 (40P/40C) | 40 (20P/20C) | 40 | Mean | 7 | B | |
| Watson155 | 2009 | 11 | NR | 54% | Laser | Inert gel | 0-10 NRS | 30 (15P/15C) | 30 (15P/15C) | 50 | Mean | 9 | W | |
| Wei157 | 2018 | 18 | 20.9 | 100% | Electrical | Sham electrode | 0-10 pain intensity | 40 (20P/20C) | 16 (8P/8C) | NA | Mean | 4 | W | |
| Weimer159 | 2019 | 78 | 27.5 | 73% | Thermal | Inert gel | 0-10 VAS | 16 (8P/8C) | 16 (8P/8C) | 30 | Mean | 2 | W | |
| Weng160 | 2021 | 32 | 22.0 | 75% | Thermal | Sham electrode | 0-10 NRS | 30 (15P/15C) | 10 (5P/5C) | 25 | Mean | 1 | W | |
| Wrobel162 | 2014 | 17 | 26.6 | 46% | Thermal | Inert gel | 0-100 NRS | 36 (18P/18C) | 30 (15P/15C) | 40 | Mean | 0 | W | |
| Wrobel161 | 2015 | 23 | 27.5 | 37% | Thermal | Inert gel | 0-100 VAS | 30 (15P/15C) | 24 (12P/12C) | 30 | Mean | 7 | W | |
| Zunhammer163 | 2018 | 33 | 25.0 | 50% | Thermal | Inert gel | 0-100 VAS | 30 (15P/15C) | 30 (15P/15C) | 40 | Mean | 1 | W |
For studies using a within-subject comparison to measure the placebo effect, N is reported as the number of participants from the group in which the comparison was made. For studies using a between-subject comparison, N is reported as the combined number of participants from the placebo and control groups from which the comparison was made, or only the placebo condition in the case of within subjects comparisons.
*
Instead of using calibrated stimulus intensities, some studies used fixed intensities, which were recorded here as NA. In the final column, “+” indicates a potential outlier with an effect size larger than the bounds of a 99% confidence interval, and “−” indicates a potential outlier with an effect size smaller than the bounds of a 99% confidence interval, based on inspection of funnel plots (Supplementary Materials, http://links.lww.com/PAIN/B752).
C, control; IV, intravenous; N, sample size; NA, not applicable; NR, not reported; NRS, numeric rating scale; P, placebo; TENS, transcutaneous electric nerve stimulation; VAS, visual analogue scale.