noun which may take the genitive case particle 'no', na-adjective, noun
Used for actions or selections made without deliberate intent, often in formal or statistical contexts. The 'unintentional' nuance is less common than 'random'.
無作為に選んだ数字で宝くじを買った。
I bought a lottery ticket with randomly chosen numbers.
調査では無作為に抽出した1000人にアンケート
In the survey, we conducted a questionnaire with 1000 randomly selected people.
作為的 means 'intentional' or 'contrived', the opposite of 無作為.
Compound of 無 (without) and 作為 (intentional action). The word 作為 itself is from Chinese, and 無作為 follows the pattern of negating a noun with 無.