Validity, Bias, Confounding

Question 1

External validity

Answer 1

Meaning of the study to the intended population

• are results of study applicable to the real population?
• are subjects comparable to the population?
• is environment/exposure comparable or achievable in nature?
• do findings relate to animals in the larger population?

Question 2

Internal validity

Answer 2

Correct assessment of exposure and outcome in study groups

- concerns the formation of the comparison groups and measurement of exposure and disease

Question 3

______ validity requires ____ validity, but not vice versa

Answer 3

External; internal

- ex: lab studies may have high internal validity, but limited external validity

Question 4

Bias

Answer 4

Systematic error that results in an incorrect estimate in the relationship between exposure and disease

Question 5

What is an example of observation bias?

Answer 5

• scale that always reads 15 lbs underweight
• doing a careful physical exam on exposed, and an abbreviated exam on nonexposed
• in assessing response to 1 of 2 abx, you know which animals received your preferred abx

Question 6

What is an example of selection bias?

Answer 6

Assigning the sickest animals to receive the new abx and the less sick your favorite abx

Question 7

Bias renders the results ___

Answer 7

Worthless!

Question 8

Selection bias

Answer 8

Concerns the animals who participate and those that do not

• if the study involves treatment, who receives what treatment
• do the animals reflect the target population -> external validity

Question 9

Information bias

Answer 9

Assessment of exposures and outcomes

- how is it measured

Question 10

Comparisons must be drawn from the same population

Answer 10

Selection bias

- loss to follow up

Question 11

Any bias that relates to data collection

Answer 11

Information bias

• observer bias
• recall bias
• misclassification

Question 12

Differences in way information collected from E+ vs E-, or D+/-

Answer 12

Observer bias

• observers are different between groups
• observers have knowledge of group, treatment, or prior findings
• observers rely on self training or experience
• observers have a conflict of interest

Question 13

Recall bias

Answer 13

Those D+ may tend to recall exposures in greater detail than those D-

Question 14

Misclassification

Answer 14

When either D or E is not properly classified

- disease is diagnosed by producers, not veterinarians

Question 15

Non-differential misclassification bias

Answer 15

Misclassification affects both D+ and D- to the same relative degree
- biases toward OR=1 (no effect)

Question 16

When does differential misclassification happen?

Answer 16

When misclassification errors are uneven between outcome groups
- unequal misclassification could produce an exaggerated RR (further from 1)

Question 17

Confounding

Answer 17

Mixing of effects

Question 18

Confounding variable

Answer 18

Extraneous variable that can wholly or partly account for an apparent association between an exposure and outcome

• produces spurious association or masks real association
• correlated with explanatory and response variables

Question 19

Confounders are related to both _____ and _____

Answer 19

Exposure and the outcome

- mixing up of things effecting outcome cannot be separated and results cannot be interpreted

Question 20

Restriction

Answer 20

Restrict study subjects to avoid known confounding factors

• vaccination is prevention of disease –> restrict to cases occurring after immunity could be operational
• get rid of animals that are at extremely high risk for the outcome, but not necessarily anything to do with exposure

Question 21

Matching

Answer 21

Match pairs on a confounding variable so that confounding is distributed evenly between comparands

Question 22

Analysis

Answer 22

• stratification

- multivariable techniques

Question 23

A large difference in crude RR and strata specific RR indicates ____

Answer 23

Confounding