Lecture 26: Understanding Study Results Flashcards Preview

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1

2 parts to interpreting the clinical relevance of
treatment effects

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

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”

alternate definition of Evidence-Based Medicine

3

dichotomous vs continuous variables

• 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)

4

3 common scenarios of experimental treatment

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.

5

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

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

6

Absolute Risk

define
how to calculate it?

§ 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

7

Absolute Risk Reduction (ARR)

define
how to calculate it?

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

8

The ARR for developing a blood clot was 5%

translate into sentence

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.

9

Number Needed to Treat (NNT)


define
how to calculate it?

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

10

NNT translation

NNT=20

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.

11

Relative Risk (RR)

define
how to calculate it?

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

RR = EER/CER

12

RR translation

RR = 0.75

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

13

Relative Risk Reduction (RRR)

define
how to calculate it?
how is it expressed?

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...

14

Relative Risk Reduction (RRR) translation

RRR = 0.25

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

15

Odds Ratio (OR)

define
how to calculate it?

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

16

when is OR close to RR?

• OR will approximate relative risk if event occurrence is rare

• Odds ratio of less than 1 represents less frequent events in the treatment group

17

Odds Ratio (OR)

translation

OR = 0.71

The odds of developing a DVT while wearing
compressions socks are about three quarters compared to the control group.

18

Line of no effect on forest plot

what is the value for
ARR?
RR?
OR?

ARR = 0%
RR = 1
OR = 1

19

Hazard Ratio

define
how to calculate it?

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

20

Hazard Ratio


line of no effect value?

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