González-Romá, V. and Espejo, B. (2003) “Testing the middle response categories "Not sure", "In between" and ‘?’ in polytomous items”, Psicothema, 15(Número 2), pp. 278–284. Available at: https://reunido.uniovi.es/index.php/PST/article/view/8120 (Accessed: 3 July 2024).