# Sampling Epidemiology Flashcards

Epidemiological approaches require?

that we gather information about populations

- understand info at pop level, particualrly health status

- we can not sample everyone in apopulation - that is impossble, so we must sample.

When you want to conduct a study, what exactly do you need to start?

You need to have a question. Some examples of the types of questions are:

1. Establish level of occurrence of a disease e.g. prevalence

2. Test hypotheses

* has the level of disease changed (Changes in level of occurrence)

* Identification of risk factors associated with disease. Sample in a way we can find differences in disease among various groups.

3. Detection of disease

* Particularly important to trade and Declaring freedom from disease.

- a lot of rules and standards for a country to declare that they are free of a disease.

How are you going to measure disease within a population?

- What’s going on in the population?
- What’s the best way to get this information?
- Sample every animal? (aka a census) If you have everyone in the population you do not need statistics. Very powerful, expensive, and difficult to do.
- Sample a portion of the animals?

- What’s the best way to get this information?

Define a census

- Census
- Every individual in the population

is sampled - Exact measurement of what is

going on

Define a sample.

- Sample
- Take a sample from the population
- Make inferences about the population based on information in the sample
- Quicker, less expensive, and practical!

How would you define target population.

- Population you want to make inferences about with respect to your objectives. E.g. Feral cats in US and Canada

Population you are trying to understand.

Source population

- Subset of the target population from which you will draw your sample. E.g. Feral cats on Long Island

Taking information from that to develop a study population.

Study population

- Individuals actually measured or enrolled in your study. E.g. Feral cats spayed or neutered by LIU’s Team in 2021

Samples you can actually get your hands on.

Define internal validity.

- Whether or not the study results (obtained from the study sample) are valid for the source population.

Define external validity.

- How well the study results can be generalized to the target population
- Subjective assessment of whether or not the source population and sample are broadly representative of the target population

Tends to refer to the reader and they are wondering is this study relevant to me. Do I care about the results of this study? Does it impact me?

Define sampling frame.

- Complete list of all sampling units.
- Required for most probabilistic sampling methods.

Define sampling unit.

- Unit of interest…
- Animals (usually the unit in vet med)
- Litters
- Pens
- Farms
- etc. …

What are the two approaches to sampling?

- Two approaches to sampling

1. Probability sampling

2. Non-probability sampling

Define probability sampling.

- Probability sampling

* Every element in the population has a know probability of being included in the sample. Where stats and assumptions hold true.

* Formal process of random selection used to ensure representativeness of the sample.

* Statistical inference can be made about the population from which you drew your sample.

Name the types of probability sampling.

- Five types of probability sample:

1. Simple random = the easiest one. (also called SRS in stats).

2. Systematic random

3. Stratified

4. Cluster

5. Multistage

Every element in the population has a known non-zero probability of being included in the sample.

Define simple random probability sampling.

- AKA Simple Random Sample (SRS)
- Forms the basis of most probability sampling methods
- Every subject has equal probability of being included in the sample
- Requires a list of all subjects (sampling frame)
- Not always easy or possible
- Procedure
- Generate list of all animals
- Assign a unique number (ID) to each animal
- Generate a list of random numbers assigned to IDs

What are the limitations of simple random probability sampling? Provide an example.

- Limitations
- Difficult to obtain a complete sampling frame
- Costs may be higher than other methods

Define systematic random probability sampling.

- Systematic random
- Does not require sampling frame, but need all animals presented sequentially
- E.g. fish processing, or appointment book (or old folder filing system)
- Do not necessarily need to know the number of sampling units (but need a rough guess)

Describe the procedure used for systematic random probability sampling. Provide an example. (see slideshow)

- Procedure
- Select your sampling interval, j
- Randomly select a number within your sampling interval
- Sample every j th animal after that

What are the limitations of systemic random probability sampling.

- Limitations
- Could be subject to bias in the interval
- E.g. (not a great example, but) if you have 5 pens and the producer takes one cow at a

time from each pen in a sequence for them to run through a chute… you shouldn’t

sample every 5th animal going through the chute!

Define stratified random probability sampling.

- Stratified random
- Sampling frame is divided into groups (strata) of defined common characteristic
- Breed, sex, age (parity), etc.
- Randomize within each stratum
- Simple random or systematic random
- Proportional to the total number, within the stratum
- Ensures all strata are present and properly represented in the study

What is an example of stratified random probability sampling. (see slideshow)

- E.g. You want to estimate the prevalence of a disease in a population of cats
- Suspect Siamese cats have a much higher prevalence of disease than DLH/DSH cats
- Want a fair representation of breeds in your study to prevent bias in estimate of prevalence