4/
Extreme case: you have only 1 treated and N₀ controls.
The true variance is σ₁² σ₀²/N₀.
But with only one treated, you just don’t have enough info to estimate σ₁² using only 1 treated!
Robust SEs simply set σ̂₁² = 0! 😵💫
σ₁²: var of treated
σ₀²: var of control