Answer: Because all individuals in the sample have the same job, so the sample is biased.
Step-by-step explanation:
Ignoring the fact that 30 is a small sample for a large city, we already know that all the people in the sample has the same profession.
Now, we can expect that two dentists win have around the same income, but not all the residents in the city are dentists
A lot of them work in fast food, others may be lawyers, others may be freelancers.
We have a lot of jobs where the income does not coincide with the average income of a dentist, so with the sample of 30 dentists we are not actually representing all the other possible jobs that there are in the city.
If we want a sample that represents the income of all the residents, we should have a random sample (so the sample is not biased, like in this case).
