Lecture 26: Understanding Study Results

Question 1

2 parts to interpreting the clinical relevance of

treatment effects

Answer 1

1. Quantify the magnitude of the treatment effect
2. Determine how precise the estimate of treatment effect is
   • Confidence intervals tell us how precise the estimated effect is

Question 2

“The use of mathematical estimates of the risk of benefit and harm, derived from high-quality research on population samples, to inform clinical decision making in the diagnosis, investigation or management of individualized patients”

Answer 2

alternate definition of Evidence-Based Medicine

Question 3

dichotomous vs continuous variables

Answer 3

• Dichotomous Variables
§ Yes / No
§ Alive / Dead
§ MI / No MI
- can be categorical

• Continuous Variables
§ Age, body weight, height, temperature
§ Student grades
§ Health-related Quality of Life scores (e.g., Pain)

Question 4

3 common scenarios of experimental treatment

Answer 4

1. When the experimental treatment REDUCES the risk of a
   BAD event.
2. When the experimental treatment INCREASES the risk of a
   BAD event.
3. When the experimental treatment INCREASES the
   probability of a GOOD event.

Question 5

what are indications of how large benefits are? (6)

Answer 5

§ Absolute Risk Reduction (ARR)
§ Relative Risk (RR)
§ Relative Risk Reduction (RRR)
§ Hazard Ratio (HR)
§ Odds Ratio (OR)
§ Number Needed to Treat (NNT)

Question 6

Absolute Risk

define
how to calculate it?

Answer 6

§ Simplest measure of association
§ Report the risk of blood clots in the
treatment group (i.e., experimental
event rate; EER)
§ Report the risk of blood clots in the
control group (i.e., control event
rate; CER)

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

Question 7

Absolute Risk Reduction (ARR)

define
how to calculate it?

Answer 7

Difference between the proportion who had an event in control group and the proportion who had an event in treatment group
• ARR = CER – EER
• ARR = c/c+d – a/a+b
• ARR = 0.20 - 0.15

= 0.05

Question 8

The ARR for developing a blood clot was 5%

translate into sentence

Answer 8

English translation: At the end of the trip, compression stockings reduced the absolute rate of blood clots by 5 percent in comparison to not wearing stockings.

Question 9

Number Needed to Treat (NNT)

define
how to calculate it?

Answer 9

The number of patients who must receive the therapy during
a specific period of time produce one good outcome
• Related to the ARR
§ NNT = 1/ARR
§ NNT = 1/0.05
§ NNT = 20

Question 10

NNT translation

NNT=20

Answer 10

English translation: You need to treat 20 patients with

compression stockings for the duration of their trip to prevent 1 person getting a blood clot.

Question 11

Relative Risk (RR)

define
how to calculate it?

Answer 11

The risk of events among patients in treatment relative to that in control

RR = EER/CER

Question 12

RR translation

RR = 0.75

Answer 12

• English translation: The risk of developing a blood clot when using compression stockings is three-quarters of that in the control group

Question 13

Relative Risk Reduction (RRR)

define
how to calculate it?
how is it expressed?

Answer 13

An estimate of the proportion of baseline risk that is removed by the
therapy
§ RRR = (CER – EER)/CER
RRR = 1 -RR

Commonly expressed as a percentage (i.e., multiply above result by 100)
§ The greater the RRR, the more beneficial therapy is…

Question 14

Relative Risk Reduction (RRR) translation

RRR = 0.25

Answer 14

Compression stockings reduced the risk of blood clots by 25% compared to control.

Question 15

Odds Ratio (OR)

define
how to calculate it?

Answer 15

The odds ratio is the ratio of the odds of the event occurring in the treatment group compared to the control group

• OR = Odds event occurs in treatment = (a/b)
______________________________________
Odds event occurs in control = (c/d)

OR = ad/bc

Question 16

when is OR close to RR?

Answer 16

• OR will approximate relative risk if event occurrence is rare
• Odds ratio of less than 1 represents less frequent events in the treatment group

Question 17

Odds Ratio (OR)

translation

OR = 0.71

Answer 17

The odds of developing a DVT while wearing

compressions socks are about three quarters compared to the control group.

Question 18

Line of no effect on forest plot

what is the value for
ARR?
RR?
OR?

Answer 18

ARR = 0%
RR = 1
OR = 1

Question 19

Hazard Ratio

define
how to calculate it?

Answer 19

HR = Hazard in Experimental Group/Hazard in Control Group

Hazard Ratio
• Relative risk over time, difference b/w curves
• Weighted RR over time during the entire study

Question 20

Hazard Ratio

line of no effect value?

Answer 20

HR = 1

When hazards of events are same in experimental and control group, hazard ratio is 1
Intervention has no effect

Less than 1 = less bad events in those with treatment