4. SURVEILLANCE AND SAMPLING Flashcards
What are public health surveillance systems?
ongoing collection and timely analysis, interpretation, and communication of health information of public health action
most of these activities are mandated by law
What are the purposes of public health surveillance systems?
to estimate magnitudes of problems
determine geographic distribution of illness
detect epidemics/define a problem
monitor changes in infectious agents
evaluate control measures
generate hypotheses and stimulate research
detect changes in health practices
facilitate planning
What are the 4 key characteristics of public health surveillance systems?
- timeliness
- representation
- sensitivity
- specificity
What is the purpose of timeliness in surveillance systems?
to implement effective control measures
What is the purpose of representation in surveillance systems?
to provide an accurate picture of the temporal trend of the disease
What is the purpose of sensitivity in surveillance systems?
to allow identification of individual persons with disease and to facilitate treatment, quarantine, or other appropriate control measures
What is the purpose of specificity in surveillance systems?
to exclude persons not having disease
What is passive surveillance?
voluntary or mandatory reporting of disease cases but there is no outreach to identify missed disease cases
provider initiated
What is active surveillance?
actively seeking cases to esnure completeness and extensive outreach to identify missed disease cases.
usually survey’s are involved
provider or health department initiated
What are the 6 planning steps for a surveillance?
- identify a health problem
- develop case definition
- collect data
- analyze data
- interpret and disseminate results
- evaluate and improve surveillance system
What is population sampling?
a method by which subjects in a given population are selected as representatives of the entire population
What are the advantages of population sampling?
less expensive
controls sampling variability
reduces sampling bias
allows application of statistical theory to results
What are the disadvantages of population sampling?
there is a likelihood of non-sampling errors like an error in measurement or errors in processing/analyzing the data
there is a likelihood of a low response rate
What are the 2 types of population sampling?
- Non-probability samples
2. Probability samples
What are non-probability samples?
when members of the population are selected in some non-random manner
What are probability samples?
when every member of the population has an equal probability of being included in the sample
What are the 4 types of sampling under non-probability sampling?
- convenience sampling
- judgement sampling
- quota sampling
- snowball sampling
What is convenience sampling?
a method for selecting study subjects when it is convenient for coordination
example: shoppers at a mall
What is judgment sampling?
method for selecting study subjects based on an investigator’s perception that sampled persons will be representative of the entire population
example: choosing one neighborhood to study
What are the 5 different types of probability sampling?
- simple random sampling
- systematic sampling
- stratified random sampling
- cluster sampling
- multistage sampling
What is simple random sampling?
n subjects are randomly chosen without replacement from a population of n subjects
each subject in the population has an equal chance of being chosen in the sample
requires enumeration of all potential subjects before sampling
What is a sampling unit?
the subjects - residents, students, patients, and etc. for which information is collected
What is a sampling frame?
a list of all those within a population who can be sampled and may include households, classes, or hospitals in the population
What is this an example of - selecting one person at random from your class?
Simple Random Sample
- finite population = class
- sampling frame = list of students
- sampling unit = student
What are the advantages of simple random sampling?
its simple to implement and understand
permits easy calculation of means and variances
What are the disadvantages of simple random sampling?
it does not provide most precise estimates
requires knowledge of sampling frame in advance
What is often used instead of simple random sampling? why?
Systematic Sampling because it is as good as simple random sampling as long as the sampling frame does not contain any hidden order
What is systematic sampling also called?
an nth name selection technique
What are the 5 steps for systematic sampling?
- determine # of subjects in population
- decide on the n (sample size) that you need
- choose sampling fraction (n/N) = interval size
- select random # between 1 and (k) to identify first subject
- then take every kth unit
What are the advantages of systematic sampling?
it does not require knowledge of sampling frame in advance
simpler to implement under field conditions (liek sampling every 5th house on the block)
What are the disadvantages of systematic sampling?
provides poor mean estimate cyclical trends in data
cannot estimate sampling variance for a single sample unless one assumes sample is same as SRS
How is stratified random sampling used?
used to ensure representation of specific subgroups of the population in overall sample
often used to reduce sampling variation
What is stratified random sampling when referred to as proportional allocation?
individuals are chosen such that proportion of individuals from given stratum in a sample is the same as proportion of individuals in the population
What is stratified random sampling when referred to as disproportionate stratified sampling?
when subgroups may be over-sampled to provide sufficient sample size for sub-analysis
What does over-representation mean?
that members of a small group of interest have a higher change of selection than everyone else
What is stratified random sampling?
divides population into strata which are groups of similar subjects
The BRFSS is an example of what kind of sampling? why?
stratified random sampling because it accounts for each state within the country and each region within a state
What are the advantages of stratified random sampling?
increased precision if strata are related to outcome of interest
can control sample size in each strata
What are the disadvantages of stratified random sampling?
requires knowledge of sampling frame in advance
When is cluster sampling used?
used when sampling unit is a cluster of subjects
What is preferable in cluster sampling?
a large number of small clusters are preferable to a small number of large clusters
What are the advantages of cluster sampling?
more convenient method when no sampling frame is available
may be more economical than SRS since sampling units are closer
What are the disadvantages of cluster sampling?
less precise than other sampling methods
special software required to compute variance, standard error, and etc.
What is multi-stage sampling?
A combination of the other sampling schemes
What is the first stage for multi-stage sampling?
define primary sampling units (like counties)
select probability sample of units, using any sampling scheme
What is the second stage for multi-stage sampling?
define secondary sampling units (like census tracts)
select probability sample of secondary units in each primary sampling unit using any sampling scheme
What is the third stage for multi-stage sampling?
define tertiary sampling units (like households)
select a sample of tertiary sampling units within each secondary sampling units using any sampling scheme
What is the fourth stage for multi-stage sampling?
to either
- select all subjects in each tertiary sampling units
- select one subject in each tertiary sampling unit
What is weighting in sampling?
corrects for differences in the probability of selection due to non-response and non-coverage errors
adjusts variables of age, race, and gender between the sample and the entire population
allows for generalization of findings to whole population, not just those who respond to the survey
What is the advantage of weighting in sampling?
allows comparability of data
What are the disadvantages of weighting in sampling?
can only be performed when the sampling methodology is carefully controlled
must use special software
