Question: what does the little u mean in statistics
[{'answer': 'a random variable', 'provenance': [{'wikipedia_id': '27590', 'title': 'Standard deviation', 'start_paragraph_id': 2, 'start_character': 26, 'end_paragraph_id': 2, 'end_character': 43, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '27590', 'title': 'Standard deviation', 'start_paragraph_id': 31, 'start_character': 0, 'end_paragraph_id': 31, 'end_character': 47, 'bleu_score': 0.8070557274927981, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 456088099802953783}, 'section': 'Section::::Definition of population values.\n'}]}]