# Shared Flashcard Set

## Details

Sampling
Who, How, Size, Why, and what
52
Science
10/05/2014

Term
 Census vs Sample
Definition
 With a census, the whole population is sampled, versus a portion of the pop'n
Term
 Why is taking a census too difficult
Definition
 time, expense, tedious, impractical, need proper participation and volunteering, and we sacrifice quality for quantity.
Term
 What are the three stages to sampling?
Definition
 1. Determine who/what to sample2. Determine how you're going to choose these subjects3. Determine how many you'll need to be confident in your findings
Term
 Your test subjects must be _____ ___________ of your population of interest
Definition
 truly representative
Term
 Give an example of inclusion and exclusion criteria
Definition
 Inclusion: Selecting individuals who you want/need to study/observeExclusion: ruling out individuals who would confound the study results
Term
 What are the different population types as we go from what we want to test, to what we actually study?
Definition
 Target PopulationSource PopulationSampling FrameSampling UnitsStudy Population
Term
 What the broad types of sampling strategies in Epidemiology?
Definition
 Non-probability, Probability, and Others (either non- or probability)
Term
 What are the types of Probability Sampling?
Definition
 Simple Random SamplingSystemic Random SamplingStratified Random SamplingCluster SamplingMultistage Sampling
Term
 What are the types of Non-probability sampling?
Definition
 ConvenienceJudgmentPurposive
Term
 What is Convenience sampling?
Definition
 Sampling units are chosen b/c they're easy to get
Term
 What is judgment sampling?
Definition
 the investigator chooses what he or she deems to be representative of the population
Term
 What is Purposive sampling?
Definition
 Sampling units are chosen on purpose because of their exposure or disease status
Term
 T or F: Non-probability sampling is a bad method
Definition
 false! It is good, it just needs caution - can produce biased results if the subjects you select are not representative of the target population
Term
 What are the pros to doing non-probability sampling?
Definition
 - relatively easy- can be cheaper if you choose subjects based on convenience- appropriate for homogenous population
Term
 In simple random sampling, it is required that the _____ _____ is known
Definition
 Sampling frame
Term
 T or F: In SRS, all individuals have an equal chance of being included
Definition
 True!
Term
 How does systematic random sampling work?
Definition
 A sampling interval (say, every 5 cows) is computed, and the starting point is selected on a formal random basis.Then, simple or systematic random sampling is conducted within each strata
Term
 T or F: In St.R.S., the percentage sampled in each strata must be the same in all groups
Definition
 false
Term
 How does stratified random sampling work?
Definition
 The sampling frame is broken into groups (strata) based on some factor that is likely to influence the level of the characteristic being measured.Then, simple or systematic random sampling is conducted within each strata
Term
 What is cluster sampling?
Definition
 The sampling unit is a group of individuals with things in common (herd, household, geographic region, etc.) BUT - the unit of concern is still the individual
Term
 T or F: Cluster sampling may be probability or non-probability
Definition
 True
Term
 T or F: In cluster sampling, not all the sampling units are tested
Definition
 False, they all are
Term
 When is multistage sampling convenient?
Definition
 - When there are too many individuals ina cluster to obtain measurements on- when the individuals in a cluster are so alike that measuring just a few will provide sufficient info- if sampling is done randomly, it can be more cost efficient than other probability sampling meathods
Term
 What is multistage sampling?
Definition
 Similar to cluster sampling, though sampling takes place at the cluster/group level as well as the individual level. (i.e. first sample random groups, then randomly sample individuals from each of those groups)
Term
 Main difference between multistage and cluster sampling?
Definition
 Multistage: Selection occurs at two levels, and only a proportion of individuals are selected and measuredCluster: All individuals in each group are selected and measured
Term
 What is the sampling unit in cluster sampling?
Definition
 The group of individuals (the herd, the barn, etc)
Term
 What is the unit of concern in cluster sampling?
Definition
 The individual (i.e. cow, child, etc.)
Term
 What is precision?
Definition
Term
 On what does precision depend?
Definition
 Sample size, variability of characteristics, and sampling strategy used.
Term
 What does a smaller CI say about the precision
Definition
 A smaller is range is more precise, and therefore better
Term
 How do you calculate sample size for estimating simple characteristics?
Definition
 1. Estimate variance2. Select level of confidence that your estimate will include the true value in the population3. Specify desired precision4. Use appropriate formula
Term
 p*q =
Definition
 The variance (of proportions) where p = diseased inds where q = non-diseased inds(p = 1-q)
Term
 what is the variance of the means?
Definition
 Σ(x – x)^2/n
Term
 What is the Z-value for a 95% confidence interval?
Definition
 1.96 (for a two-sided hypothesis)
Term
 What is another way to describe precision?
Definition
 L, the margin of error
Term
 In a descriptive study, why do we calculate sample size?
Definition
 To estimate the mean or a proportion (different equations)
Term
 If you decrease the allowable margin of error, what will happen to the size of the sample that you require?
Definition
 It will drastically increase
Term
 How do you calculate sample size in analytic observational studies?
Definition
 1. State null & either 1- or 2-sided alternative hypothesis2. Determine what it is you're comparing3. Determine how much of a difference b/w the groups you want to detect and if necessary, the expected variance4.Set alpha and beta (confidence and power)5. Use the appropriate formula or table (based on what you're comparing) to estimate sample size
Term
 Define Power
Definition
 (P) The ability of a test to detect differences between groups when a real difference exists
Term
 What is a commonly used power, and what is the Z-value?
Definition
 80% power, beta = 0.2Z(0.2) = -0.84
Term
 What is a type 1 error?
Definition
 When we reject the null, given that it's true
Term
 What is a type 2 error?
Definition
 When we don't reject the null, given that it's false
Term
 What is the probability of making a type 1 error?
Definition
 alpha
Term
 What is the probability of making a type 2 error?
Definition
 beta
Term
 Most hypotheses are...
Definition
 Two sided
Term
 What is a twp sided hypothesis?
Definition
 When you predict that a value can either be higher or lower in one group over another
Term
 As the size of the difference between 2 means or proportions decreases, the required sample size....
Definition
 increases
Term
 As the level of power desired to detect a difference between the two groups increases, the required sample size....
Definition
 also increases
Term
 As the number of confounders you're controlling for increases, the required sample size....
Definition
 also increases
Term
 As the number of hypotheses tested increases, the required sample size....
Definition
 also increases
Term
 What is a better method of representing the population that you want to characterize, prob sampling or non-prob sampling?
Definition
 Prob sampling, though it may not always be practical or possible
Term
 What is required to calculate an appropriate sample size?
Definition
 - desired precision of your estimate- expected variation in the data- (in analytical studies) the size of difference you want to detect b/w groups; and the desired power
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