Legal analogy transfers a ruling by its effective cause. That cause may be known through the Quran, Sunnah, consensus or qualified inference.
In correspondence of the same type, irrelevant differences cannot defeat the analogy. In correspondence by genus, the common category and specific distinctions require testing. In inferred-cause reasoning, suitability and evidentiary strength matter.
The practical outcome is an analytical method: identify the original and new cases, establish the cause, assess any operative difference and check superior evidence. These tests distinguish a defensible qiyas from a superficial comparison.