This case study was made in the context of a tutorial on regularisation methods, which presentation is available in January_2019_Tutorial_Presentation__Regularisation_.pdf.

It includes an introduction on Ordinary Least Squares (OLS) regression, the motivations behind regularisation as well as the different interpretations (optimisation, geometric, Bayesian) for common regularisation methods:

• Ordinary Least Squares (no regularisation)
• Lasso (L₁) regularisation
• Ridge (L₂) regularisation
• Elastic Net (mixture of L₁ and L₂)
• Bridge (L_p) regularisation

Further methods are then discussed to overcome overshrinkage:

• Hybrid Lasso (Lasso followed by OLS)
• Relaxed Lasso (Lasso followed by Lasso)
• Horseshoe and regularised horseshoe

The different methods are compared in a simulation study to evaluate how they fare for different datasets, in the presence of multicollinearity or low/high signal-to-noise (SNR) ratio.

Results are described in January_2019_Tutorial_Presentation__Regularisation_.pdf. The code of the analysis is available in main.R. The code for the regularised horseshoe model is available in regularised_horseshoe.stan.