Flashcards in Lecture 26: Understanding Study Results Deck (20)

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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

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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

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