Lecture 27: Understanding Study Results Cont'd

Question 1

what are indications of how large risks are? (4)

Answer 1

• Absolute risk increase (ARI)
• Relative Risk (RR)
• Relative Risk Increase (RRI)
• Number Needed to Harm (NNH)

Question 2

Absolute risk

definition
how to calculate?

Answer 2

§ Report the risk of swelling & ischemia in the treatment group (i.e., EER)
§ Report the risk of swelling & ischemia in the control group (i.e., CER)

EER = a/a+b
CER = c/c+d

Question 3

Absolute Risk Increase (ARI)

definition
how to calculate?

Answer 3

• Difference between the proportion who had an event in treatment group and the proportion who had an event in control group
• ARI = c/c+d – a/a+b
• ARI = EER – CER

Question 4

Absolute Risk Increase (ARI)

= 0.01

translation

Answer 4

Wearing compression stockings increased

the absolute risk of getting swelling/ischemia by 1%

Question 5

Number Needed to Harm (NNH)

definition
how to calculate?

Answer 5

• The number of patients who must receive the therapy for one additional patient to be be harmed during a specific period of time

• Related to the ARI
§ NNH = 1/ARI
§ NNH = 1/0.01
§ NNH = 100

Question 6

Number Needed to Harm (NNH)
= 100
translation

Answer 6

You need to treat 100 patients with
compression stockings for the duration of their trip to produce
1 extra episode of swelling/ulceration

Question 7

Relative Risk (RR)

how to calculate?

Answer 7

Relative Risk = EER/CER
= 2/100
1/100

• RR = 0.02 / 0.01
• RR = 2

Question 8

Relative Risk (RR)
= 2

translation

Answer 8

§ The risk of developing swelling/ischemia was 2 times greater in those who wore compression stockings
§ The risk of developing swelling/ischemia was doubled by compression stockings

Question 9

Relative Risk Increase (RRI)

definition
how to calculate?

Answer 9

An estimate of the proportion of baseline risk that is added
by the therapy
§ RRI = (EER – CER)/CER
§ RRI = (0.02 – 0.01)/0.01
§ RRI = 1

• Alternatively RRI = RR-1 = 2-1 = 1 (expressed as 100%)

Question 10

Relative Risk Increase (RRI)
= 1

translation

Answer 10

Compression stockings increased the risk of

edema/ischemia by 100%.

Question 11

odds ratio for risk

how to calculate

Answer 11

OR = ad/bc
= 299/981
= 2.02

eg. The odds of leg ischemia was doubled

Question 12

Absolute measure used for

Reduces A BAD Event
Increases a BAD event
Increases a GOOD event

Answer 12

Reduces A BAD Event: Absolute Risk Reduction
Increases a BAD event: Absolute Risk Increase
Increases a GOOD event: Absolute Benefit Increase

Question 13

relative measure used for

Reduces A BAD Event
Increases a BAD event
Increases a GOOD event

Answer 13

relative risk for all

odds ratio for all

Question 14

relative CHANGE measure used for

Reduces A BAD Event
Increases a BAD event
Increases a GOOD event

Answer 14

Reduces A BAD Event: relative Risk Reduction
Increases a BAD event: relative Risk Increase
Increases a GOOD event: relative Benefit Increase

Question 15

numbers needed to ______ measure used for

Reduces A BAD Event
Increases a BAD event
Increases a GOOD event

Answer 15

Reduces A BAD Event: Number Needed to Treat
Increases a BAD event: Number Needed to Harm
Increases a GOOD event: Number Needed to Treat

Question 16

what 2 NNT limitations are there?

Answer 16

1. Baseline Risk - NNT Depends Heavily On Baseline Risk

2. Time Frame - NNT is an Inherently Time Dependent Measure

Question 17

1. Baseline Risk - NNT Depends Heavily On Baseline Risk

Answer 17

high risk patients and low risk pts - if they have the same RRR, the NNT for high risk patients is much lower

eg. NNT is 10 for high risk and 200 for low risk pts

Relative risks - don’t need to know characteristics of a pop that much

Absolute risk and NNT highly sensitive to baseline risk

Question 18

2. Time Frame - NNT is an Inherently Time Dependent Measure

Answer 18

• The NNT depends on WHEN the outcomes are counted

• Are these NNTs all equal?
§ 30 over 5 years
§ 150 over 1 year
§ 15 over 10 years

• NNTs become more impressive (i.e., decrease) as the
duration of follow-up increases
Talk about 10 years for more impressive NNT

Question 19

when is RRR best used?

Answer 19

When people do not specify what they are talking
about…
§ e.g., Drug therapy was 30% effective in reducing the risk of death
§ …Likely talking about RRR as it gives the most “impressive” numbers
§ The relative efficacy of drug therapies is often similar across patient subgroups
at different risk & usually is assumed to be constant over time.
• Applying across diseases, various time points, personalizing risk
calculations;

Question 20

Communicating with health care providers; not ideal for

communicating with patients

Answer 20

NNT

Question 21

what’s best for communicating with patients?

Answer 21

EER; CER; ARR:

EER, CER, ARR, NNT: All highly dependent on baseline risk & time point!