ABSTRACT
Participant variability is traditionally looked upon as introducing extraneous variance to the data collected in an otherwise highly controlled experiment. Such variables include momentary states (such as how tense, relaxed, hungry, tired or agitated a person feels), but also broad demographic characteristics (such as their birth cohorts, national/ethnic backgrounds, or first languages) as well as other characteristics that describe a person (such as social attitudes, motivations, personal traits, habits and other enduring behaviour patterns). Humans are diverse, and researchers will always be at the mercy of participant variability, which is why relevant participant variables need to be identified and appropriately considered when planning and conducting research. This chapter presents three strategies to handle participant variables: approaches that seek to reduce participant variability, such as selection procedures or homogenising procedures; approaches to systematically address participant variables, such as using them for stratified randomisation or adding them as planned experimental or quasi-experimental factors; and approaches to register variability in participants, such that it can be reported and statistically controlled for. Each strategy will be discussed along with the different kinds of participant variables, employing a specific example from the experimental linguistics literature.
