Arguments by analogy are determined to be strong or weak based on:

1. the number of similarities between the two cases
2. the relevance of the similarities between the two cases
3. the number of dissimilarities between the two cases

Unless two cases are identical (in which case they are the same thing) there will be a set of differences between them. I.e., there is no such thing as a perfect analogy. This is why all arguments by analogy are inductive by nature.

See Informal Fallacy.