Sampling Flashcards
Mastery
Target population vs sample
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How to determine sample size…
What is the purpose of sampling?
* Access information on the population of interest in a
reasonable / do-able fashion
* Help eliminate selection bias
* Account for natural variable in ethnicity, sex, education,
etc. that exist in the population of interest
* Check in on our subjects to see how they are
progressing, could be an aspect of our testing… **
What is the purpose of sampling?
* Access information on the population of interest in a
reasonable / do-able fashion
* Help eliminate selection bias
* Account for natural variable in ethnicity, sex, education,
etc. that exist in the population of interest
* Check in on our subjects to see how they are
progressing, could be an aspect of our testing… **
Sampling Bias: (what we are trying to minimize!)
* Occurs when the individuals selected for a sample….
* Can be either … or …
* To prevent / protect against sampling bias: typically, an…
- Usually carried out on sample of subjects rather than whole populations.
- Ideally drawing a random sample from the target population to which the results of the study would be generalized.
- Sampling bias occurs in almost all studies to a lesser or greater degree.
- Degree of bias based on understanding of the population from which the sample was drawn.
- Generalizing to a larger population depends on this understanding
Sampling Bias: (what we are trying to
minimize!)
* Occurs when the individuals selected for a sample over- represent or under-represent certain population attributes that are related to the phenomenon under study
* Can be either conscious or unconscious
* To prevent / protect against sampling bias: typically, an impartial mechanism is required to make unbiased
selections of a sample (enforce randomness)
- Usually carried out on sample of subjects rather than whole populations.
- Ideally drawing a random sample from the target population to which the results of the study would be generalized.
- Sampling bias occurs in almost all studies to a lesser or greater degree.
- Degree of bias based on understanding of the population from which the sample was drawn.
- Generalizing to a larger population depends on this understanding
Target Population/Accessible Population
* Target population refers to overall group of people (or objects) in which…
* Accessible population refers portion of the target population that has a…
Target Population/Accessible Population
* Target population refers to overall group of people (or objects) in which the researchers intend to generalize the findings about
* Accessible population refers portion of the target population that has a chance of being selected into the study
- Probability Samples
- Nonprobability Samples
Non-probabilistic sampling. Why would you do it?
4 types…
* You are:
* Seeking participants with…
* Seeking participants who have a particular…
* You need to select people within a…
* People who are willing to talk…
- Probability Samples
* This is done through the process of random
selection
* Everyone has an equal chance or equal “probability”
of being chosen - Nonprobability Samples
* Made through nonrandom methods
- Seeking participants with a particular trait, feature, disease, condition, ethnic origin, etc.
- Seeking participants who have a particular vantage point, life experience, who are of a certain age, share a lived experience, etc.
- You need to select people within a group (community or otherwise) to study it
- People who are willing to talk speak about experiences
- Inclusion Criteria: refers to the primary traits of the target and accessible populations that will qualify someone as a subject
- Are these factors important to the question being studied?
- What inclusion criteria will be important in your proposed study?
- Exclusion Criteria: refers to those factors that would exclude someone from being a subject
- These factors will generally be considered potentially confounding to the potential results of the study
- What exclusion criteria will be important in your proposed study?
- Convenience sampling: Chosen on basis of availability
*
*
* - Snowball
* Participants identify and recommend other potential participants who are…
* …ferred (I told …) - Purposive
* Maximum…
- Homogeneous Purposive Sample (sample …)
- Typical Case Sampling
- Extreme Case Sampling
- Critical case Sampling
- Total Population Sampling (extremely difficult as…)
4. Expert Sampling
- Convenience sampling: Chosen on basis of availability
* Requests for volunteers
* Recommended by others
* Volunteered - Snowball (similar to convince sampling but more selective)
* Participants identify and recommend other potential participants who are deemed fitting to the research purpose
- Referred (I told two friends and so on and so on.
- Purposive (sampling with a defined purpose)
* Maximum Variation/Heterogeneous Purposive Sample
* Homogeneous Purposive Sample (sample one type)
* Typical Case Sampling
* Extreme Case Sampling
* Critical case Sampling
* Total Population Sampling (extremely difficult as population
expands) - Expert Sampling
*turn over to the specific group. give 4 people who are best fit for this, from an expert
Randomness
* Mathematical – probability
* Equal likelihood of being selected
- Random Selection (… validity)
- Random selection from…
- Random Assignment (… validity)
- If you are comparing groups, then…
- Mathematical – probability
- Equal likelihood of being selected
- Random Selection (external validity)
- Random selection from the population of interest
- Random Assignment (internal validity)
- If you are comparing groups, then random assignment to groups
internal validity(how well it relates to my IV, trustworthy cause and effect relationship)
Simple Random Sampling
* Use of a random numbers table
* Randomly pick a number and travel in any direction to get remaining
numbers
* Computer generated random numbers
* E.g. Lotto quick pick
* Names in a hat…
- Sample size (n = 5; n = 10; n = 20)
- Random Selection
- Random Assignment
- Sample size (n = 5; n = 10; n = 20)
- Random Selection External Validity
External validity refers to the extent to which the results of a study can be generalized to other settings, populations, and times. It determines whether findings from a particular study apply beyond the specific conditions under which the study was conducted. - Random Assignment Internal Validity
Internal validity refers to the extent to which a study establishes a cause-and-effect relationship between the independent and dependent variables, without interference from confounding factors. It ensures that changes in the dependent variable are genuinely caused by the independent variable, rather than by extraneous variables.
Sampling Techniques - Probability
Sampling (popular in quantitative studies)
Random selection
- Randomized sample
- Systematic sample
*
* - Stratified random sample
*
* - Clustered Sampling
*
- Randomized sample
- Systematic sample
* Random start
* Every nth person or object - Stratified random sample
* Breakdown by characteristic/category
* sub populations - Clustered Sampling
* Break population into clusters and randomly select
the cluster
Subtill difference
*What is the likely hood of flipping 5
heads in a row with a “fair coin”
*There is a difference in probability
for events when they are:
*Predicted
*Experienced
