Objective: Baseline testing is used in amateur and professional sports as a component of comprehensive concussion management programs. We provide information on the base rates of low scores on ImPACT that will facilitate the interpretation of baseline and post-injury neurocognitive testing. Participants and Methods: Participants were 46,679 adolescents and young adults who completed baseline computerized neurocognitive evaluations with ImPACT. They had valid baseline scores, and subjects with self-reported ADHD or academic problems were not included. The breakdown of the sample by gender-stratified age cohorts was as follows: females aged 14-18=14,426 and 19-22=7,640, and males aged 14-18=14,487 and 19-22=10,126. ImPACT is a brief computer-administered test battery. The four primary composite scores used for the base rate analyses were Verbal Memory, Visual Memory, Processing Speed, and Reaction Time. Results: The base rates of low scores in adolescents and young adults on the four composite scores vary by level of cutoff and do not conform to a standard normal distribution. Having one or more scores at or below the 2nd percentile occurs in 7-8% of the subjects in the four age cohorts, and having one or more scores at or below the 10th percentile occurs in 26-28% of the subjects in the four age cohorts. Obtaining two or more scores below the 16th percentile occurs in approximately 15% of adolescents and young adults. Age and gender stratified base rate results for the following cutoff scores are provided: 25th, 16th, 10th, 5th, and 2nd percentiles. Conclusions: When multiple neurocognitive test scores are interpreted simultaneously, the metrics of the standard normal distribution cannot be relied upon. That is, approximately 15% of healthy subjects will obtain a score that is greater than one standard deviation below the mean, when a single score is considering in isolation, but 40% will obtain one score in this range if the four scores are considered simultaneously.