Add Missingness Indicators - Missing Values, Exactly

Indicator columns keep the replacement visible to later methods.

highlighted = computed this step

Add age_missing

Row B had a missing age, so age_missing is 1 for B. Rows with observed age get 0.

B_{\text{age missing}}=1

Row C had a missing color, so color_missing is 1 for C. Rows with observed color get 0.

C_{\text{color missing}}=1

The filled value and the missingness indicator are both present. The replacement is not silently erased.

\text{filled value}+\text{missingness indicator}