The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 l/min and standard detion 20 l/min :
S.D AREA BETWEEN S.D One S.D 68.3% Two S.D 95.4% Three S.D 99.7% So, 95%, about girls have PEFR between two S.D Given in question mean = 300L/min S.D = 20L/min A. Girls between one S.D= 68.3% = 300+/- 1 x20 = 320 to 280 So, 68.3% girls have PEFR between 320 and 280L/min B.Girls between two S.D = 95.5% = 300+/- 2 x 20 = 340 to 260 So, 95% girls have PEFR between 260 and 340L/min C. Girls between three S.D = 99.7% = 300 +/- 3 X 20 = 300+/- 60 = 360 to 240 Ref: Park's Textbook Of Preventive And Social Medicine By K. Park, 19th Edition, Page 402
About 95% of the girls have PEFR between 260 and 340 l/min
The girls have healthy lungs
About 5% of girls have PEFR below 260 l/min
All the PEFR must be less than 340 1/min