Python is a bit under-equipped for statistical modeling

One of the course participants was trying to fit a linear mixed effects model with two levels of grouping. [`statsmodels.MixedLM`](https://www.statsmodels.org/stable/generated/statsmodels.regression.mixed_linear_model.MixedLM.html#statsmodels.regression.mixed_linear_model.MixedLM) does not support this. You can interface with R packages that provide this functionality through [pymer4](https://eshinjolly.com/pymer4/), but installing that in a Jupyter notebook is nontrivial because it also involves installing the dependent R packages.

Take home point: you can do statistical modeling in Python, but if your use case is somewhat advanced, you are probably better off using something more special-purpose such as R. We should add this as a note to the statistical modeling section in the tips.

在 tips 中找到統計建模部分，並閱讀其周圍關於 Python 建模的指引。新增一則註記，說明 statsmodels.MixedLM 對兩個分組層級的限制，以及 R 等更專業工具的適用性，然後確認該註記是否適合放在此部分中，並準確反映所連結的資源。

由索引模型根據 Issue 內容生成。