Formulas

Question 1

Define population prevalence (P-Prev)

Answer 1

(TP + FN)/Total Pop x 100%

• Known positives/total population

Question 2

Define apparent or test prevalence (A-Prev)

Answer 2

(TP+FP)/Total Pop x 100%

• All test positives/Total Population

Question 3

Define sensitivity

Answer 3

TP/(TP + FN)x 100%

• % of ‘Known’ positives that the test detects correctly

Question 4

Define specificity

Answer 4

TN/(TN + FP) x 100%

• % of ‘Known’ negative animals that the test detects correctly

Question 5

Define Positive Predictive Value (PPV)

Answer 5

TP/ (TP + FP) x 100%

• % of test positives that are actually infected

Question 6

Define Negative Predictive Value (NPV)

Answer 6

TN/ (TN + FN) x 100%

• % of test negatives that are actually healthy

Question 7

Define accuracy (ACC)

Answer 7

(TN + TP) / Total population x 100%

• % of the population that was correctly sorted using the test

Question 8

Define Point Prevelance (PP)

Answer 8

# with the disease/total population at risk AT A SPECIFIC TIME

Question 9

Define Period Prevelance

Answer 9

# with disease at any point/ total population at risk OVER A SECTION OF TIME

Question 10

Define the incidence or incidence rate (IR)

Answer 10

• # of new cases/population at risk OVER A SECTION OF TIME
• does not include already infected individuals in either numerator/denominator
• incidence = risk

Question 11

How would the denominator of your Incidence Rate equation change if you’re dealing with a fluid population?

Answer 11

= (Pop. at risk at start of time + Pop. at risk at the end of the time)/2

* but if population is reasonably stable, just use starting population

Question 12

What is the formula for Mortality Rate?

Answer 12

of deaths in the population over a specific time/ population at risk over the time period

Question 13

What is the formula for Case Fatality?

Answer 13

of deaths due to the disease/# of diseased individuals

Question 14

What is the formula for survival?

Answer 14

(# of cases - # deaths due to the disease)/# of cases

• ie. # that survive/# that were sick

Question 15

What is the formula for Relative Risk (RR)?

*Only used in cohort studies

Answer 15

• Risk in the exposed/risk in the non-exposed
• (Exposed sick/total exposed) / (Non-exposed sick/ total non-exposed)

Question 16

What is the formula for an Odds Ratio (OR)?

*cohort or case-control study

Answer 16

• odds that a case was exposed/odds that a control was exposed
• (Sick exposed/ sick nonexposed) / (Healthy exposed / healthy non-exposed)

Question 17

What is the formula for Risk Difference (RD)?

Answer 17

• Risk in the exposed - risk in the non-exposed
• (Exposed sick/ total exposed) - (Non-exposed sick/ Total non-exposed)