A sports scientist wants to investigate whether performing a 45-minute highly-focused cognitive endurance task affects a person's auditory reaction time.
To do this, the scientist measured the reaction times of 8 volunteer university hockey players in response to a sudden auditory beep.
Each player then completed the 45-minute computerised cognitive endurance task.
Immediately after finishing, the scientist measured their auditory reaction times again under identical quiet laboratory conditions.
Give three ways that the scientist could improve this investigation to obtain more valid and reliable results.