Convenience sampling
Convenience sampling selects whichever population units are easiest to reach, such as passers-by or a class of students, instead of using random selection, risking an unrepresentative sample.
See it move
A coffee chain manager surveys the first 60 people who pass a single city-centre branch at lunchtime, out of 40 stores nationwide — convenience sampling, chosen only because they happened to be there. Simple random sampling instead draws individuals with a known, equal chance from the full customer base. Because convenience sampling uses no random mechanism, its results carry no valid margin of error.
Check yourself
A gym manager wants to estimate the average number of weekly visits among all 3,000 members. He asks the 40 members who happen to be at the gym during his own Monday 6pm shift. Which sampling method has he used, and what is the main risk?