Throughout your research it is of utmost importance to define and understand your specific outcome(s) of interest. In other words, you should clearly define what you want to measure, i.e. the outcome or construct of interest. This outcome should be clearly defined, so you are able to select an appropriate outcome measurement instrument measuring the specific outcome, judge whether the items, task, observations or parameters of an outcome measurement instrument are relevant for the outcome, and whether the outcome is comprehensively covered by the items/task/observations/parameters. Others should be able to understand what you want to measure. Just a word describing the outcome of interest may not be enough.
For example, if you want to measure pain, it is important to define what aspect of pain you intend to measure, e.g. pain intensity, pain interference, etc. If this is not described, it is hard to select the best suitable PROM. Complex multidimensional outcomes, such as quality of life, contains multiple different (unidimensional) subdomains. The relevance of different subdomains may differ across diseases, resulting in quality of life outcome measurement instruments measuring different subdomains, and subsequently, defining quality of life in different ways.
To define your outcome, a conceptual framework is useful as this defines levels of health, and differentiates between levels and outcomes. Subsequently, relationships between clearly defined (preferably unidimensional) outcomes can be studies.
Several frameworks are developed, such as the International Classification of Functioning, Disability and Health (ICF), the framework developed by Wilson and Cleary (1995), or the Filter 2.0 framework developed by OMERACT.