The PEFR of a group of 11 year old girls follow a normal distrubution with mean 300 1/min and standard detion 20 limin -

Ans. is 'a' i.e., About 95% of the girls have PEFR between 260 and 340 1/min o 1 SD includes --> 68% of values o 2 SD includes --> 95% of values o 3 SD includes --> 99.7% of values In this question Mean PEFR = 300 L/min SD = 2 L/min i) Area around 1 SD on either side of mean (x 1 SD) will include 68% of values, i.e. x+-1SD=300+-20 So, 68% of girls have PEFR between 280 & 320 L/min. ii) Area around 2SD on either side of mean (x 2SD) will include 95% of values, i.e. x+ 2SD=300+-40 So, 95% of girls have PEFR between 260 & 340 L/min. iii) Area around 3SD on either side of mean (x 3SD) will include 99.7% of values, i.e. - x+-3SD =300+ 60 So, 99.7% ofgirls will have PEFR between 240 & 360 L/min.