Description: Everyone in the population has an equal chance of being selected. It usually consistes of pulling names out of a hat or a random generator.
Advantages: No bias in selection, your results should be more representative, doesnt take very long unless sample is massive- thus more cost efficent.
Disadvantages: The larger the population the more difficult it becomes. You cannot be certain that your sample will representative all groups i.e. you may randomly pick 10 people with brown hair, 5 people with blond hair and no black haired people. In some experiments this mayt cause your results to not give a fair representation of the population.
Example: A univeristy undertook a study of how long students spend working each week. They gave out the survey to a group of 50 students, randomly selected from acroos the university by using a random computer generator.
Description: This is where the researcher takes the sample from the people who are available at the time who fit the necessary criteria.
Advantages: Quick and inconvient, also more ethical as researcher can more easily ju7dge if a particpant is too busy to take part or whever they will be able for the study.
Disadvantages: Very unrepresentative of the population as researcher will pick people who they believe will take part anf give good results, as this is natural human instinct. This would create bias results. People may also decline to take part, thus turning your sample into a volunteer sample- which would most likely produce quite different results.
Example: You are going to study the effect of violent tv on teenagers. You thus watch the first 20 teens you find, and analyze the effect of violent tv on them over a certain period of time.
Description: This is where people volunteer to take part in the study after seeing an advert of being asked.
Advantages: Ethical as the poarticipants choose to take part. They are also more likely to co-operate with the reserayhc as they have chosen to take part so should be more mnotivated.
Disadvantages: Biased as the participant may be more motivated, which would produce different results. They may also change their behaviour to suite the study- this is know as demand characteristics. It may also take a long time to find enough willing participants.
Example: A study to find out whever taking omega 3 supplements will increas iq. The researcher would ask people whever they were willing to take these and see if their iq increased.
Description: The target population is divided into subests such as age, gender and eye colour and a representative sample of each is found.
Advantages: Provides best cahnce of unbiased sample and limits the number of participants needed by using criteria.
Disadvantages: Can become quite complicated if lots of sub-sets are involved, increasing the necessary time to complete the study. It also relies on the researcher knowing all the required groups, which can lead to bias by excluding people.
Example: If you have 100 people, and 70 of them are males and 30 females, in a stratified sample of 20 there will be 14 males and 6 females.