'control' or eliminate , in the sense of 'exclude the effects :
1- is by making them constant.
2- randomise the variable
3- 'stratified sampling
4- 'matched subjects design
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وإذا فشلنا بازالة التأثير ,, يصبح البحث 'confounded' design
If you fail to make sure relevant variables are controlled, then you may have what is called a 'confounded' design