noun, noun which may take the genitive case particle 'no'
below average; substandard
Used to describe something that falls below a standard or average level, often in evaluations or comparisons.
His grades were below average.
The quality of this product is substandard.
平均以下 means 'below average' in a statistical sense, while 標準以下 implies falling below a set standard or norm.
Compound of 標準 (standard) and 以下 (below, less than).