1) The manner in which the sample for the study is chosen; how the sample is selected will affect the outcome of the study.
Randomized selection: Each subject has an equal chance of being selected i.e., every nth person on a list. Used to reflect characteristics such as gender, age, or income in the sample as they appear in the population; can project the results of the sample to the population.
Convenience selection: Subjects are selected based on availability, not based on chance as in a randomized selection; cannot project the results of the sample to the population.
2) The process of measuring certain conditions in the environment (i.e., air samples).