90% can be below average. It depends on how you define 'average.' If you're using the median, then 'below average' can only be 50%. But if you use the mean (or

90% can be below average. It depends on how you define 'average.' If you're using the median, then 'below average' can only be 50%. But if you use the mean (or its variants), then you can have 90% of a population be 'below average' if the population is very skewed.