A convenience sample of cars parked near the gate may give a biased estimate of the proportion of cars which are from a Japanese Manufacturer.

Different investigators may get different estimates for the proportion of cars that are red.

The sample proportion of cars that are red is an unbiased estimate of the population proportion if the sampling method is simple random.

A sample of 100 cars in a convenience sample is always better than a sample of 20 cars from a proper random sampling method.