The PEFR of a group of 11 year old girls follow a normal distribution with mean 300 L/min and stardard detion 20 L/min-
Ref:Parks 23rd edition pg 849. SD = 1 : confidence limit = 68% SD = 2 : confidence limit = 95% SD = 3 : confidence limit = 99.7% Here 2SD = 2x20 = +/-40 So, between 260 and 340 2SD covers 95% values, remaining is 5%, So 5% fall below 260 and above 340 Precisely 2.5% below 260 and 2.5% above 340
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 L\/min
Biostatistics