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 中找到统计建模部分，并阅读其周围关于 Python 建模的指导。添加一条注释，说明 statsmodels.MixedLM 对两个分组层级的限制，以及 R 等更专业工具的适用性，然后验证该注释是否适合放在该部分中，并准确反映所链接的资源。

由索引模型根据 Issue 内容生成。