Literature AppraisalLiterature Appraisal
Effectiveness of TherapyEffectiveness of Therapy
Measures of treatment effect
• Statistical significance• Odds ratio• Relative risk• Absolute risk reduction• Number needed to treat
Measures of treatment effect
Outcome (death)
Yes No
Control a b
Experiment c d
200
Total
200100
100
Total in each group
200100
100
25
Die
10
Total in each group
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive)Total in each group
+
+
After 1 year
Risk & Relative Risk
200100
100
25 (75)
Die
10 (90)
(Survive) Risk? (a proportion)
What is the Total in each group
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) RiskTotal in each group
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) RiskTotal in each group
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) RiskTotal in each group
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.2525%
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25 25%
+
+
After 1 year
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25 25%
0.110%+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25 25%
0.1 10%+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
Risk ratio or Relative risk (RR) =
+
+ 0.25 25%
0.1 10%
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
Risk ratio or Relative risk (RR) =
+
+ 0.25 25%
0.1 10%
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
Risk ratio or Relative risk (RR) =
2.5
+
+ 0.25 25%
0.1 10%
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.
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds?What about
+
+
200100
100
25 (75)
Die
(90)
(Survive) Risk
0.25
0.1
Odds
10 +
+
200100
100
25 (75)
Die
(90)
(Survive) Risk
0.25
0.1
Odds
10
1to3
+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9
Odds Ratio (O.R.) =
+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9
Odds Ratio (O.R.) =
+
+
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9
Odds Ratio (O.R.) =
+
+
3
200100
100
25 (75)
Die
10 (90)
(Survive) Risk
0.25
0.1
Odds
1to3
1to9
3
Odds Ratio (O.R.) =
+
+
Risk ratio or Relative risk (RR) =
2.5
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
• Control group event rate (CER) = Deaths / Controls
• Experiment group event rate (EER)
= Deaths / Treated
Absolute risk reduction (difference)
ARR = CER - EER
Absolute Risk Reduction
200100
100
25
Die
10
Risk
0.25 25%
0.110%
Absolute Risk Reduction
(ARR) =
CER
EER
200100
100
25
Die
10
Risk
0.2525%
0.110%
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
200100
100
25
Die
10
Risk
0.2525%
0.110%
Absolute Risk Reduction (ARR) =
0.1515%
NNT=1/ARR=1/0.15=
6.7
Q.E.D.
MAGPIEOf 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
