1941
Today , the optimization of process parameters , for a given metal powder , is generally attained through an empirical trial and error approach .
 concept or principle: optimization; concept or principle: process parameters; material: metal powder; concept or principle: empirical; concept or principle: error
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: optimization, process parameters, metal powder, empirical, error 
Answer:
 concept or principle: optimization; concept or principle: process parameters; material: metal powder; concept or principle: empirical; concept or principle: error