measuring health & disease

Question 1

false + vs false −

Answer 1

false positives can be caused by:

• similar disease agent ➞ confusion
• previous exposure
• lab/test error
• ensuring low false positives important for diseases w/ $$ tx or tx that cause suffering

false negatives can occur from:

• timing ➞ early in subclinical stage ➞ might not be long enough for immune system to begin response
• immunocompromised indiv don’t produce antibodies so tests cannot pickup
• ensuring low false negatives important for highly transmissible & severe diseases with extreme consequences

      - ex: PRRS

Question 2

proportion of the population diseased and exposed

Answer 2

Question 3

proportion of the population exposed

Answer 3

Question 4

proportion of the population diseased

Answer 4

Question 5

true prevalence

Answer 5

actual level of disease prevalence in the pop
* cannot know ➞ estimate using AP

Question 6

apparent prevalence

Answer 6

what prevalence appears to be based on the test
* how many people in the pop that T+

Question 7

sensitivity

Answer 7

proportion of D+ that T+
* how accurate the test is at identifying diseased indiv
* ↑ Sn ➞ indiv that T+ are D+

Question 8

specificity

Answer 8

proportion of D− that T−
* how good a test is at identifying non-diseased indiv
* ↑ Sp ➞ indiv that T− are D−

Question 9

predictive value

Answer 9

probability of disease given the test result

Question 10

positive predictive value (PPV)

Answer 10

probability/proportion of T+ that are D+

Question 11

negative predictive value (NPV)

Answer 11

proportion of animals who T− that are D−

Question 12

↑ prevalence =

Answer 12

↑ PPV & ↓ NPV

more dis in pop ➞ more indiv will T+ so less indiv will T−

Question 13

↓ prevalence

Answer 13

↓ PPV & ↑ NPV

less dis in pop ➞ less indiv will T+ so more indiv will T−

Question 14

AP < TP

Answer 14

underestimates TP of disease ➞ more false T− so D+ are missed

Question 15

AP > TP

Answer 15

overestimates TP of disease ➞ more false T+ so D− animals are falsely dx

Question 16

what measures do we use to quantify disease in populations?

Answer 16

• amount of dis

  - morbidity: # of infxt 
  - mortality: # of deaths from dis

• temporal distribution
• geographic/spatial distribution
• prevalence
• incidence
• cumulative incidence
• approximate incidence

Question 17

prevalence

Answer 17

number of instances in a known pop at a specific point in time
* snapshot in time
* diseases with long durations
* identifying common dis in pop
* evaluating control strategies

Question 18

incidence

Answer 18

number of new cases in a known pop over a period of time
* preferred over prevalence
* dis w/ short durations
* important for:

* understanding disease development and transmission (temporality) 
* predicting risk of disease (change in health status)

Question 19

cumulative incidence

Answer 19

proportion of non-dis animals at beginning of study that become dis during the study
* aka avg risk of developing disease during a time period
* who is getting diseased over time
* risk of disease

Question 20

incidence rate

Answer 20

how quickly new cases develop over time
* closed pop only

Question 21

approximate incidence rate

Answer 21

how quickly new cases develop over time for open pop

Question 22

closed populations

Answer 22

defined for entire study ➞ no additions or loses
* more control
* less bias
* dis-free animals at start = at risk
* preferred over open

Question 23

open populations

Answer 23

animals enter/leave throughout duration of study
* hard to keep sample size ➞ difficult to track long term
* stable only if additions = withdrawals over time
* not as rigorous
* cannot calculate disease risk from open pop

Question 24

study designs

Answer 24

descriptive: describes
* summarize & describe data
* no comparisons
* hypothesis generating
* frequency & distribution
* cannot make conclusions about associations btwn E & O
1. case report: rare or new conditions/dis involving 1-2 animals
2. case series: collection of an (4+) w/ same rare condition/dis
3. descriptive surveys: to estimate freq & percentages of diff variable

analytical: compares
* explore relationships & associations
* test hypotheses
1. experimental: control treatments & manipulate envir
2. observational: natural experiments
-no control over treatment or animals
-no manipulation of envir
-focused on exposures (E) & outcomes (O)