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In [479]: df
Out[479]:
ID birthyear weight
0 619040 1962 0.123123
1 600161 1963 0.981742
2 25602033 1963 1.312312
3 624870 1987 0.942120
In [480]: df.loc[:, 'weight'].mean()
Out[480]: 0.83982437500000007
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DataFrame.mean(axis=None, skipna=None, level=None, numeric_only=None, **kwargs)
DataFrame.mean()