| msg317048 - (view) |
Author: W deW (W deW) * |
Date: 2018-05-18 19:29 |
| The 0.5-quantile or median is defined for ordinal, interval, and ratio scales. An Enumerator as derived from Enum and extended with rich comparison methods implements an ordinal scale. Therefore calculating the median over a list of such enum-elements ought to be possible. The current implementation tries to interpolate the median value by averaging the two middle observations. This is allowed for interval and ratio scales, but since this interpolation involves an addition, not so for ordinal scales. Although computationally it is possible to do this for numeric ordinal variables, logically it is non-sense for the distance between ordinal values is - by definition - unknown. On non-numeric ordinal values it is even computationally impossible. The correct return value would be: the first value in an ordered set where al least half the number of observations is smaller or equal than it. This is observation[len(observation)//2] for odd and even length ordered lists of values. Whether the same applies to interval and ratio scales is a matter of opinion. The currently implemented algorith definitely is more popular these days. |
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| msg317120 - (view) |
Author: Steven D'Aprano (steven.daprano) *  |
Date: 2018-05-19 14:18 |
| For ordinal scales, you should use either median_low or median_high. I don't think the standard median function ought to choose for you whether to take the low or high median. It is better to be explicit about which you want, by calling the relevant function, than for median to guess which one you need. |
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| msg317122 - (view) |
Author: Steven D'Aprano (steven.daprano) * |
Date: 2018-05-19 14:53 |
| By the way, this isn't a crash (that's for things which cause the interpreter to segfault). I'm marking this as Not a bug, but I'm open to suggestions to improve either the documentation or the median functions. |
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| msg317125 - (view) |
Author: Steven D'Aprano (steven.daprano) * |
Date: 2018-05-19 15:10 |
| What do you think of adding a note in the documentation for median? "If your data is ordinal (supports order operations) but not numeric (doesn't support addition), you should use ``median_low`` or ``median_high`` instead." |
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| msg317248 - (view) |
Author: W deW (W deW) * |
Date: 2018-05-21 19:04 |
| Changing the documentation in tis way seems to me an excellent and easy way to solve the issue. |
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| msg317694 - (view) |
Author: Terry J. Reedy (terry.reedy) * |
Date: 2018-05-25 17:56 |
| I agree. |
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| msg319219 - (view) |
Author: Tal Einat (taleinat) * |
Date: 2018-06-10 13:17 |
| PR ready for review. |
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| msg320414 - (view) |
Author: Tal Einat (taleinat) * |
Date: 2018-06-25 11:04 |
| New changeset fdd6e0bf18517c3dc5e24c48fbfe890229fad1b5 by Tal Einat in branch 'master': bpo-33573: docs to suggest median() alternatives for non-numeric data (GH-7587) https://github.com/python/cpython/commit/fdd6e0bf18517c3dc5e24c48fbfe890229fad1b5 |
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| msg320416 - (view) |
Author: Tal Einat (taleinat) * |
Date: 2018-06-25 11:18 |
| New changeset 150cd3cb272021e9a2d865dd28486b00199fe77d by Tal Einat (Miss Islington (bot)) in branch '3.7': [3.7] bpo-33573: docs to suggest median() alternatives for non-numeric data (GH-7587) (GH-7906) https://github.com/python/cpython/commit/150cd3cb272021e9a2d865dd28486b00199fe77d |
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| msg320417 - (view) |
Author: Tal Einat (taleinat) * |
Date: 2018-06-25 11:27 |
| New changeset 8fd8cfa369fe4b6d6ac430cd28ead32717df7bee by Tal Einat (Miss Islington (bot)) in branch '3.6': [3.6] bpo-33573: docs to suggest median() alternatives for non-numeric data (GH-7587) (GH-7907) https://github.com/python/cpython/commit/8fd8cfa369fe4b6d6ac430cd28ead32717df7bee |
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