Literature Appraisal
Effectiveness of Therapy

Measures of treatment effect
•
•
•
•
•
Statistical significance
Odds ratio
Relative risk
Absolute risk reduction
Number needed to treat

Measures of treatment effect
Control
Experiment
Outcome (death)
Yes a c
No b d

Total
200

Total in each group
100
200
100

After 1 year
Total in each group
Die
100
200
100
25
10

After 1 year
Total in each group
Die (Survive)
100
200
100
25 + (75)
10 + (90)

Risk & Relative Risk

After 1 year
What is the
Total in each group
Die (Survive) Risk?
(a proportion)
100
200
100
25 + (75)
10 + (90)

After 1 year
Total in each group
Die (Survive) Risk
100
200
100
25 + (75)
10 + (90)

After 1 year
Total in each group
Die (Survive) Risk
100
200
100
25 + (75)
10 + (90)

After 1 year
Total in each group
Die (Survive) Risk
100
200
100
25 + (75)
10 + (90)

After 1 year
100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
25%

After 1 year
100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
25%

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
25%
0.1
10%

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
25%
0.1
10%

100
200
100
Die (Survive) Risk
25 + (75) 0.25
25%
10 + (90) 0.1
10%
Risk ratio or Relative risk (RR) =

100
200
100
Die (Survive) Risk
25 + (75) 0.25
25%
10 + (90) 0.1
10%
Risk ratio or Relative risk (RR) =

100
200
100
Die (Survive) Risk
25 + (75) 0.25
25%
10 + (90) 0.1
10%
Risk ratio or Relative risk (RR) =
2.5

Odds & Odds Ratio

Odds
• The ratio between the amounts staked by parties in a bet, based on the expected probability either way.
• The balance of advantage or superiority.

100
200
100
Die
What about
(Survive) Risk Odds?
25 + (75)
10 + (90)
0.25
0.1

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3
1to9

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3
1to9

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3
1to9
Odds
Ratio
(O.R.) =

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3
1to9
Odds
Ratio
(O.R.) =

100
200
100
Die (Survive) Risk
25 + (75)
10 + (90)
0.25
0.1
Odds
1to3
1to9
Odds
Ratio
(O.R.) =
3

100
200
100
Die (Survive) Risk
25 + (75) 0.25
Odds
1to3
10 + (90) 0.1
1to9
Risk ratio Odds or Relative Ratio risk (RR) = (O.R.) =
2.5
3

Measures of treatment effect influence clinicians decisions
• Clinicians:
– more inclined to treat if the results are presented as relative risk
– less inclined to treat if the results are presented as absolute risk reduction
Forrow et al. Am J Med 1992;92:121

Absolute Risk Reduction
• Control group event rate (CER)
= Deaths / Controls
• Experiment group event rate (EER)
= Deaths / Treated
Absolute risk reduction (difference)
ARR = CER - EER

100
200
100
Die Risk
25
10
0.25
25%
0.1
10%
Absolute Risk
Reduction
(ARR) =
CER
EER

100
200
100
Die Risk
25
10
0.25
25%
0.1
10%
Absolute Risk
Reduction
(ARR) =
0.15
15%

Number Needed to Treat
NNT

Clinical value of measures of treatment effect
•
Number Needed To Treat
The odds ratio etc. not easy to understand, especially for patients.
• The number needed to treat (NNT) to prevent an adverse event is a more clinically relevant measure of the consequences of treatment
Sackett DL. EBM 1996; 1: 164-6
Sinclair JC. J Clin Epidemiol 1994; 47: 881-9

Number Need to Treat (NNT)
Out of 100 patients treated 10 died compared to 25 in the placebo group and 15 extra survived.
Therefore:
To get 1 more patient to survive,
6.7 (100/15) have to be treated.

100/15
1/ 0.15
NNT = 1/ ARR

100
200
100
Die Risk
25 0.25
25%
10 0.1
10%
Absolute Risk
Reduction (ARR) =
0.15
15%
NNT=
1/ARR=
1/0.15=
6.7

Q.E.D.

MAGPIE
Of the patients treated (5015) 40 fitted compared to 96 in the placebo group (5055)
In % Mg 0.8% vs Placebo 1.9%
Therefore: ARR 1.8 – 0.8 = 1.1%
(11 per 1000)
To get 1 more patient to survive,
91 (100/1.1) have to be treated. = NNT