Then judgment is used to select the subjects or units from each segment based on a specified proportion.
This means that individuals can put a demand on who they want to sample (targeting).
The problem is these samples may be biased in a way that is difficult to quantify or adjust for.
For example, if interviewers decide to question the first person they see, they may oversample tall respondents (who are more easily visible from a distance), which could lead to an overestimate of average income.
This non-random element is a source of uncertainty about the nature of the actual sample.