“…The construct validity was reported in all the studies except in one ( Jaafar et al, 2020 ). The dimensionality of the instruments using factor analysis was analyzed in 21 studies ( Russell, 1996 ; De Jong Gierveld and Van Tilburg, 2010 ; Durak and Senol-Durak, 2010 ; Ayala et al, 2012 ; Iecovich, 2013 ; Neto, 2014 ; Penning et al, 2014 ; Velarde-Mayol et al, 2016 ; Lee and Cagle, 2017 ; Tomás et al, 2017 ; Uysal-Bozkir et al, 2017 ; Ausín et al, 2019 ; Faustino et al, 2019 ; Cheung et al, 2020 ; Hosseinabadi et al, 2020 , 2021 ; Caycho-Rodríguez et al, 2021 ; González-Tovar and Garza-Sánchez, 2021 ; Lee et al, 2021 ; Pedroso-Chaparro et al, 2021 ; Rodríguez-Blázquez et al, 2021 ); the Delphi method in one study ( Leung et al, 2008 ); principal component analysis without rotation in 1 study ( Buz and Pérez-Arechaederra, 2014 ); principal component analysis with oblique rotation in 1 study ( Honigh-de Vlaming et al, 2014 ); the Rasch model in 1 study ( Park et al, 2019 ); and exploratory structural equation modeling in 1 study ( Sancho et al, 2020 ). Overall, the instruments are multidimensional, and thus so is loneliness, except for seven studies that conclude that the instruments are unidimensional ( Russell, 1996 ; Buz and Pérez-Arechaederra, 2014 ; Neto, 2014 ; Velarde-Mayol et al, 2016 ; Tomás et al, 2017 ; Pedroso-Chaparro et al, 2021 ; Rodríguez-Blázquez et al, 2021 ).…”