Measures of Association

Define Association

relationship b/w 2 variables that renders them statistically dependent

• positive or negative

• association does NOT equal causation

What measures are absolute measures?

• Risk Difference (RD)

  • absolute risk reduction (ARR)

  • absolute risk increase (ARI)

• Number needed to treat (NNT)

• number needed to harm (NNH)

What measures are relative measures?

• Risk Ratio (RR)

  • relative risk reduction (RRR) and relative risk increase (RRI)

• Odds Ratio (OR)

Steps in measuring association

1. summarize data in table

2. calculate measures of disease frequency

3. calculate measures of association

Table to summarize data:

How to calculate cumulative incidence of outcome among exposed/tx group

How to calculate cumulative incidence of outcome among unexposed/tx group

Define Risk Difference

measures difference in the risk (CI) of developing the disease b/w 2 groups

Harmful effect means

• absolute risk increase (ARI)

  • risk in tx group > risk in control group

  • RD is POSITIVE

Beneficial effect means

• absolute risk reduction (ARR)

  • risk in tx group < risk in control group

  • RD is NEGATIVE

No association means

risk in tx group = risk in control group

Risk Difference Equation

Define number needed to treat (NNT)

• # of individuals who would have to receive the tx for one outcome to be prevented

• NNT = 1/RD (ARR)

  • SMALLER NNT = BETTER

Define number needed to harm (NNH)

• # of individuals who would have to receive tx for one outcome to be experienced

• NNH = 1/RD (ARI)

  • LARGER NNH = BETTER

Define risk ratio

measures ratio of risk (CI)

Risk Ratio Equation

If RR < 1, the risk is ____

decreased

If RR > 1, the risk is _____

increased

If the RR = 1, then

there is no difference b/w the 2 groups

Define Relative Risk Reduction

measures extent to which tx REDUCES a risk compared to the control group

Relative Risk Reduction Equation

Define relative risk increase (RRI)

measures extent to which tx group INCREASES a risk compared to control group

relative risk increase equation

Define Odds Ratio

• measures the likelihood of an event occurring

• ratio of odds of event in tx group to control group

Odds Ratio equation

If OR < 1, the odds will be

decreased

If OR = 1, then

there is no difference in the odds

If OR > 1, then the odds will be

increased

the higher the prevalence, the more OR ____ the RR

overestimates

When the outcome (disease) is rare, then

OR ≈ RR