experiments - sampling Flashcards
difference between population and sample
population
- group of people focus of researchers interest = target population
sample
- group of people taken from target population
- assumed that sample is representative of target population
5 different sample techniques
- stratified
- volunteer
- opportunity
- random
- systematic
random sampling AO1
- every person has equal chance of being in sample
- take all names of people and write them on equal sized papers
- place them in a hat
- pick number of names wanted in the sample
random sampling strength
- unbiased
- every ppt has equal chance of being selected for study CONTEXT
- extraneous variables equally divided within group
- increases internal validity
random sampling weakness
- time consuming
- takes time collecting all names of ppts for study - CONTEXT
systematic sample AO1
- system applied to collect ppts
- put system in place to organise ppts e.g. alphabetical order
- system e.g. every 5th/10th ppt picked out until desired number reached
systematic sample strength
- avoids researcher bias
- researcher has no control over which ppts picked CONTEXT
- objective system
systematic sample weakness
- may not be representative of target population
- everyone has equal chance of being picked
- difficult to generalise to target population - CONTEXT
- may also be time consuming and difficult
stratified sample AO1
- make-up of sample reflects make-up of target population
- e.g. if 70% target population = female then 70% sample = female
HOW
1. identify sub-groups in target population e.g. male/female
2. calculate how many ppl needed for each proportion e.g. 70% of 80
3. use random sampling to decide how many ppl will make up proportions
stratified sample strength
- most representative
- reflects sub-groups of target populations
- in correct proportions
- findings easily generalised to target population
HOWEVER
- cannot reflect all ways people are different
- not perfect
stratified sample weakness
- time consuming
- takes time to calculate
- not guaranteed all ppts will take part
calculating sample sizes
- with percentage
- sample size wanted/100 x percentage size of group - without percentage
- sample size wanted/target pop size x sub-group size
opportunity sample AO1
- being there in the moment in time
- e.g. school canteen, first 30 ppl that walk past
HOW - go to place where you will have access to target population
opportunity sample strength
- very quick, easy and less money
- choose who is available at time for sample - CONTEXT
- unlike random sampling = long/lengthy process
- list of names etc.
opportunity sample weakness
- may not be representative of target population
- only people that are there at a given time
- doing similar things CONTEXT
- so they may have similar personalities
- harder to generalise to target population
