Most Important Final Exam Flashcards
What are disadvantages of a cohort study?
* Require long follow up period
* Losses to follow up can be a problem (producing non response, migration and loss to follow up biases)
* Expensive
* May require large samples
* Not suitable for rare diseases
* Not suitable for diseases with long latency
* Sampling, ascertainment and observer biases are still possible
What are advantages of case control studies?
* Efficient method for studying rare diseases
* Quick (because subjects have already experienced the outcome)
* Cheaper (because they are shorter as well)
What are disadvantages of a cross sectional study?
* No information about incidence
* Difficult to investigate cause and effect relationships (establishing correct temporal sequence can be difficult)
* Not appropriate for diseases of short duration
- milk fever in dairy cattle
- claw amputation
If in a high prevalence condition, which test is recommended: diagnostically specific or diagnostically sensitive?
Highly sensitive because there are many animals that are truly cases, it is therefore more likely you will get false negatives. So you want to minimize the false negatives as much as possible. On the other hand, there won’t be many truly negative animals, so false positives are less of an issue.
What are disadvantages for case control studies?
- Cannot provide info on disease incidence in the studied population (we’re only working with a sample of the disease negative population- we don’t know the size of the PAR!!!** we have purposely monkeyed around with the population)
** so instead of calculating risk… we can calculate odds ratio.
- Recall bias and quality of past record keeping
- Unbiased selection of control group and to assign group based on measurement of exposure
- If the incidence of exposure is high, can be difficult to show the difference between cases and controls
When might you use an odds ratio instead of a risk ratio?
If only prevalence estimates are available (i.e. in a survey or cross- sectional study) OR in a case-control study.
In these circumstances it must be recognized that the odds ratio is just an approximation (an exaggeration) of the relative risk.
What is the table with the advantages and disadvantages of all of the studies?
R2M2TIL
If ever in doubt as to use a diagnostically sensitive or diagnosticlaly specific test, what would be a good rule?
* A highly sensitive test (low fast negatives) to generate a pool of potentially diseased individuals followed by a highly specific test (low false positives) to weed out the false positives out of the first one.
* At high prevalence, the negative predictive value of a test is pretty low too so without the sensitive test– a highly specific test won’t really help you make any decent conclusions about the disease status- there is a high probability it is just a false positive
If accuracy is above 80% and all of the tests are reasonably good, do you think these are reasonable scientific conclusions? Support answer with appropriate evidence and argument.
The conclusions concerning overall accuracy appear correct. The relative differences in false positive rates (higher in Herefords), appears to have been misinterpreted. This is most likely due to a lower prevalence of JD in Herefords than Friesians. The test is reliable in both breeds, there are just some slight variations due to demongraphics and breed differences. True differences in prevalence actually exist and these are influencing PPVs.
What are the three classical conditions for considering a variable to be a confounder?
- It is causally associated with the outcome
- It is causally or non-causally associated with the exposure
- The founding variable and the exposure variable are on two separate causal pathways to the outcome
What are the advantages of a cohort study?
* Provide estimates of the absolute incidence of disease in exposed and non-exposed individuals
* exposed status is recorded before disease has been identified: provides clear information about whether exposure preceded disease
* Particularly useful for studying rare EXPOSURES
* Suitable for studying multiple outcomes
Why is it important to establish a case definition when investigating outbreaks of disease?
A case definition ensures that the outcome of interest is consistently defined across space (e.g. among different investigation centres in a large scale outbreak) and over time. Consistency in case definition is important because it will allow the incidence of disease to be reliably measured which in turn allows responses to control efforts to be monitored.
Comparison of cross sectional, case-control, and cohort studies
When is the odds ratio a poor estimator of the risk ratio?
If the incidence risk is relatively high, then OR will be a poor estimator. The OR is a reliable estimator of the RR if the incidence risk is <10%.
What are the 10 investigatory steps in an outbreak?
PV2D2-FE-CIC
- Prepare for fieldwork
- Verify the outbreak
- Verify the diagnosis
- Define, identify, and count cases
- Describe the outbreak according to individual, place, and time
- Formulate a working hypothesis
- Evaluate and test your hypothesis
- Conduct follow up investigations
- Implement control and preventive measures
- Communicate findings and make recommendations
