平均数=(8+9+10+11)÷4=9.5s²=[(8-9.5)²+(9-9.5)²+(10-9.5)²+(11-9.5)²]/4=(1.5²+0.5²+0.5²+1.5²)/4=(2.25+0.25+0.25+2.25)/4=5/4=1.25