Define the Standard Deviation (2 score) classification?

Statistics teaches us that when normal values of any variable are distributed as per their frequency of occurrence, they follow normal distribution encompassing values within two standard deviations (SD), of mean/median (Average). On this premise, all the children with weights less than median -2SD of normal weight for age (NCHS standards) are considered as suffering from under nutrition. The following criteria given in Table are used to classify children into various degree of under nutrition based on mean/median and SD.

 Table: SD classification for weight for age

You have learnt about different forms of PEM. You must be now wondering how common the occurrence of PEM in our communities is. I.e. what is the prevalence of the problem?