Sampling Flashcards
What is a representative sample?
A sample that accurately represents the population that you are studying.
What is a biased sample?
A sample that does not represent the targeted population, contains different characteristics and the procedure favors the selection of certain individuals in comparison to others.
Example of biased samples? (3)
- Researcher going door to door
- Financial compensation
- Survey return rate
What do we use to calculate minimum sample size?
Power analysis
What are the major factors that influence sample size? (4)
1) Size of the difference you need to detect
2) Variability in the factor of interest
3) P value you plan to use as a criterion for statistical significance
4) How confident do you want to be that you will detect statistical significance, assuming a difference exists
What is the effect size in the power analysis?
The difference between the mean (quantitative) or proportions (qualitative) of the two groups.
What is Standard deviation?
The variability within a sample.
What is a Type 1 error?
P level, usually 0.05 (5%).
What is power in the power analysis?
The probability of finding an effect (“how confident do you want to be of your effect”), usually 80%.
What is the direction of the effect in power analysis?
It can only go up in one tail; up and down in two-tail.
What are the types of Statistical tests? (2)
The chi-square, t-test, etc.
What is attrition in power analysis?
An adjustment in sample size (eg. death of animals, dropout)
What is the Resource equation?
E = (Total number of animals) − (Total number of groups)
What is the E variable? (resource equation)
The degree of freedom of analysis of variance (ANOVA)
When do you use the resource equation? (4)
When you cannot predict:
1) Effect size (ie. mean).
2) Standard deviation (ie. no previous findings available).
3) Multiple endpoints measured.
4) Complex statistical procedure used.
When do we use non-probability sampling?
When the population is not completely known.
What are the two types of non-probability sampling?
1) Convenience sampling
2) Quota sampling
What is convenience sampling?
A sampling method where participants are easy/convenient to access/reach and are available to participate.
What is quota sampling?
Identify a subgroup, then choose from each subgroup to get a quota.
What is simple quota sampling?
Sampling method where little is known about the characteristics of a target population and equal number of participants selected for.
What is proportionate quota sampling?
Sampling method where demographics are known and proportionately represented in sample.
What is animal research quota sampling?
Sampling method where buying animals from single supplier and thus animals obtained have same proportion on many characteristics.
What is the pro of animal research quota sampling?
Provides a sample that is representative of the target population.
When do we use probability sampling?
When the target population is know, specifically the size.
What are the four probability sampling methods?
1) Simple random
2) Systematic
3) Strata
4) Cluster
What is simple random sampling?
Sampling method where all individuals have an equal chance of being selected and each selection is independent of others.
Example: Choose 10 students from this class, give all students a number, then pick 10 numbers.
What is simple random sampling with replacement?
After each selection, he/she is added back into the pool and an individual can be chosen more than once.
What is the warning (caveat) for simple random sampling?
When participant selection is left completely up to chance, can lead to a sample that is not representative of the population.
What is systematic sampling?
Sampling method where you choose every # participant on a list. Combination of simple random sampling + systematic process.
What is stratified sampling?
Sampling method where the population is divided into strata (subgroups), then participants selected from each strata.
What is simple stratified sampling?
Obtain equal numbers from each strata:
* Equal representation
* Does not represent proportions in the population
What is proportional stratified sampling?
Select a different proportion (%) from each strata:
* Proportion depends on what representation in the actual population.
* Difficult to compare between strata if some strata makes up a small percent of the population.
Quota vs Simple stratified sampling? Similarities?
Subgroups are grouped together.
Quota vs Simple stratified sampling? Differences?
The difference is in random selection
-Quota sampling – not randomly selecting individuals from
the population. Researcher chooses who gets sampled (individuals are pre-selected out of convenience)
-Stratified sampling – randomization first then selection
What is cluster sampling?
Sampling method where the population can be identified into clusters; then choose from a proportion of the clusters.
Advantage of cluster sampling? (2)
1) Quick and easy to get a large sample
2) Can measure all individuals in groups
The disadvantage of cluster sampling?
Some clusters are completely omitted, what if something is unique about certain clusters?
