A new type of pain reliever is administered to 30 consenting post-operative patients in various hospitals.Although the pain reliever has already been tested for safety and effectiveness,this experiment is to observe and categorize any side-effects.Because of maturity and body mass,it is decided to test the adults separately from the children. 
The grouping of the adults separate from the children is an example of what?
Definitions:
Variable Costs
Expenses that vary in relation to the amount of products or services a company generates.
Fixed Costs
Expenses that do not change with the level of goods or services produced by a business over a certain period.
Net Income
The total earnings of a company after all expenses and taxes have been subtracted from total revenue.
Sales
The revenue generated from goods or services sold by a company in the ordinary course of its business.
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