To mistake the part for the whole or draw conclusions from incomplete data. This idiom comes from a story about blind men who each touch a different part of an elephant and describe the animal based only on what they felt.

(idiom) blind people touch an elephant; to be unable to see the big picture or to draw a conclusion based on incomplete information

(idiom) a group of blind people touch an elephant, and each describes a different part as the whole; to be unable to see the big picture because of a limited perspective

(idiom) to touch an elephant with eyes closed; to act blindly or without proper understanding of the situation