L7 sampling
- 1. Connect What different groups are there within ISVA List as many as you can think of
- 2. Learning Objectives Define 4 sampling techniques Practice 3 of these using Skittles Evaluate the different sampling techniques
- 3. Big Picture • Takes too long to study everyone Population • Use a sample of the population to represent everyone Sample • Key word: representative
- 4. Expert Input Different sampling techniques Stratified Random Volunteer Opportunistic
- 5. Student Led How to do it? Follow the instructions on the worksheet You will need a calculator! Colour Number Red Ask if you need help Green Yellow Orange Purple
- 6. Discussion Method Procedure Strength Weakness Random Sampling Stratified Sampling Opportunity Sampling Volunteer sampling
- 7. Sampling procedures: Simple random sampling • Simple random sampling: E.g. pulling names out of a hat. Though psychologists and sociologists tend to number all the names in the sampling frame, then get a computer to randomly select the numbers. It is better to say this in exam than say names out of a hat.
- 8. Strength and weakness Low bias because everone has equal chance of being chosen so you cant knowingly or unknowingly introduce a bias by eg putting the better looking and perhaps more confident people in 1 condition/group if confidence was a factor in your d.v Sample can be checked mathematically for any bias - for true random sampling you need a perfect list for your sample frame on a practical level this is almost impossible to achieve even for people with a huge amount of funding ie even the electoral register is not a perfect list of everyone in the uk.This can cause bias as those missing may be a specific type of people.(eg homeless or those with mental health problems or non comforming types.) - Even if you have the perfect list once you have made your random choice of participants it is very unlikely that you will get access to everyone you originally choose .(again bias potential as it is particular type of person that refuse to be involved)
- 9. Volunteer self selecting Asking people to volunteer. If questionnaires are sent out by post those returning them would e considered volunteers. Milgram used volunteers and he paid them for their time,
- 10. Strengths and weaknesses Ethically good because people have freely put themselves forward to take part You are more likely to get full cooperation - Only certain types of people may volunteer and this is likely to cause a serious bias in the results and limits the generalisability of the findings in certain types of experiments especially those of a social nature - It may take a long time to get enough volunteers in comparison to an opportunity sample
- 11. Opportunity sample This is when the researcher takes who ever is available. If the researcher asks someone directly to take part and they agree this is an opportunity sample and not a volunteer sample. If a general advert is put out and people put themselves forward then that would be a volunteer sample
- 12. Strengths and weaknesses It is ethical as long as pp given full information of the study they are asked to participate in also the experimenter can judge if the person is likely to be upset an only target suitable participants A good sized sample can be obtained far more quickly than a volunteer or random sample - Using only who is available may mean you only have a certain group of people eg mums at home or students who visit the library - Open to the experimenter choosing a specific group ie people they are confident to approach
- 13. The sampling frame is divided into ‘strata’. A random sample is then taken from each stratum .You generate categories that may be relevant to the nature of the study such : Stratified sampling as age gender ethnicity and then ensure you have a certain number from each. I t may be 400 a number proportional to the sample frame students 50% male 50% female 200 males 200 females 75% white 25% ethnic minority 75% white 25% ethnic minority 50 ethnic minority 150 white 50 ethnic minority 150 white males males females females 5 ethnic 15 white 15 white minority 5 ethnic males females males minority
- 14. Strengths and weaknesses All the relevant groups or strata have at least some representation Limits the number of participants needed compared with random sampling to be sure of having some representaion from each group/strata - The researcher is really unlikely to be aware of every group and sub group so you are attempting the impossible and in doing so may create a bias by missing out key groups and even over representing certain groups that have similarities which is less likely to happen in a random sample - It is difficult to get a truly representative number for each group and could end up with a bias in terms of proportions