literature appraisal effectiveness of therapy
DESCRIPTION
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. Outcome (death) YesNo Control ab Experiment cd. Total. 200. - PowerPoint PPT PresentationTRANSCRIPT
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