Research

Probability sampling is the likelihood of any one member of the population being selected is known. 

Nonprobability sampling is the likelihood of selecting any one member from the population is not known.

Because the selection of participants is determined by chance. Therefore, the chance that the sample will truly represent the population is increased.

-It is the most common type of Probability sampling procedure. 

-Each member of the population has an equal and independent chance of being selected to be part of the sample.

• 1- Definition of the population from which you want to select the sample
• 2- Listing of all the members of the population
• 3- Assignment of numbers to each member of the population
• 4- Use of a criterion to select the sample you want

-Is the most unbiased tool you can use to select participants from a population.

To use a criterion that is unrelated to that which you are studying. 

ex. if you are doing a study on volunteering, you do not want to ask for volunteers!

-It reduces the chances of certain participants being selected; therefore, it is less unbiased than simple random sampling.

-It is a preferred technique because it is easier and less trouble than random sampling yet less precise.

Process of selecting a sample that represents different groups or levels of a population.

-It is used to ensure that the strata (or layers; ex. race, gender etc) in the population are fairly represented in the sample.

-A random sampling procedure used when subjects are known to be unequal on some variable in the population.

-Units of individuals are selected rather than individuals themselves. 

-It is a great time saver but be sure that the units are homogeneous enough such that any differences in the unit itself might not contribute to a bias.

1-Convenience Sampling: based on convenience 

• 2- Quota Sampling: Selects people with the characteristics you want (such as first grade, rural children) but does not randomly select from the population.
• -However, a specified number is set to be selected and once that number is met, no further selection occurs.