A researcher wants to study the effects of caffeine on concentration levels. To gather participants, the researcher invites $15$ colleagues from the office to join the experiment. Identify the sampling technique used, and discuss any potential sources of bias.

Cluster sampling, which might introduce bias if the chosen cluster does not reflect the entire population

Convenience sampling, bias may occur because of the lack of diversity and non-random selection of participants

Stratified sampling, with bias possibly occurring if the strata are not representative of the population

Simple random sampling, bias may arise if the selected individuals have similar habits