Week 3 - Epi II

Question 1

What is Sensitivity?

Answer 1

Sensitivity (Sn):

• correctly identify as diseased those actually with disease.
  
  • Positive in Disease
• Is 1 – false neg rate.
• Denominator = condition present

Use a high Sensitivity to rule out

• Snout: If 100% Sensitive, all (-) are True Negatives

Question 2

What is Specificity?

Answer 2

Specificity (Sp):

• correctly identify as non-diseased those actually without the disease
• Negative in Health
• 1 – false pos rate
• Denominator = condition absent

Use a high Specificity to Rule in

• Spin: If 100% Specific, all (+) are True Positives

Question 3

What is Predictive Value (+)?

Answer 3

Predictive Value (+):

• proportion true + of those screening as +
• Denominator = test positive

Probability that person actually has disease, given a pos test result

Question 4

What is Predictive Value (-)?

Answer 4

Predictive Value (-):

• proportion true - of those screening as –
• Denominator = test negative

Probability that person actually not have disease, given neg test result

Question 5

What are the 3 ways to measure Risk?

Answer 5

1. Absolute Risk
   
   • Incidence of disease in group initially free of condition
   • new cases ratio
     
     • new cases/group
2. Attributable Risk
   
   • Incidence of disease attributable to exposure
   • subration
     
     • exp - non-exp
3. Relative Risk
   
   • How many times more likely exposed develop disease relative to non-exposure
   • division
     
     • exposed/non-exposed

Question 6

Number Needed to Treat (NNT) vs. Number Needed to Harm (NNH)

Answer 6

Number Needed to Treat (NNT)

• NNT = 1 / Absolute Risk reduction
• # needed to tx to prevent 1 case

Number Needed to Harm (NNH)

• 1 / Absolute Risk increase
• # needed to tx to cause 1 extra case

Question 7

When should we use Relative Risk (RR)?

Answer 7

Cohort studies

• 2 groups to start:
  
  • exposed & not exposed
  • cases/conditions develop or appear over time

RR determines how much more likely exposed develop disease compared to non-exposed

————————

How to calculate:

RR = (a/a+b)/(c/c+d)

Question 8

When should we use an Odds Ratio (OR)?

Answer 8

Case Control studies

• 2 groups to start:
  
  • cases & not cases (controls)

OR determines relative freq of exposure among cases and non-cases

• Odds that a case is exposed divided by odds that a control is exposed

————————

How to calculate:

OR = ad/bc

• If OR >1 = freq of exp higher among cases thus increased risk
• If OR <1 = freq of exp lower among cases, thus decreased risk or ‘protection’

The stronger the association, the higher the OR

Question 9

Continuous vs. Categorical Variables

Answer 9

Continuous = T-test (group 1 mean vs. group 2)

• Appropriate for quantification
• Infinite scale
  
  • body weight
  • height
  • BP in mm Hg
• If limited to integers —> Discrete
  
  • # cigs/day
  • # children
  • visits
  • days

Categorical = Chi Squared (group A vs group B)

• Not appropriate for quantification
• Classified, categories, bins
  
  • BMI as ↑ or ↓
  • Height as tall or short
  • BP as hypo or hypertensive
  • Race, Gender, Eye Color
• If 2 possible values → Dichotomous
  
  • dead or alive
• If categories not ordered → Nominal
  
  • blood type
• If categories ordered → Ordinal
  
  • mild, moderate, or severe