This chapter discusses the two major theories underlying scale development: classical test theory, which has dominated the field for the past century, and item response theory, which is more recent. It begins by summarizing the history of measurement, first of physical and physiological parameters and later of intelligence. This is followed by the steps involved in developing a scale: creating the items, determining if they fully span the construct of interest while at the same time not including irrelevant content, and assessing the usability of the items (whether they are understood correctly, whether they are free of jargon, if they avoid negatively worded phrases, etc.). The chapter then describes how to establish the reliability and validity of the scale—what are called the psychometric properties of the scale. It concludes by discussing some of the shortcomings with classical test theory, how item response theory attempts to address them, and the degree to which it has been successful in this regard. This chapter should be useful for those who need to evaluate existing scales as well as for those wanting to develop new scales.
Reliability and validity of PSQI among frontline health care workers of COVID-19 using classical test theory and item response theory