Interpreting Study Results

Question 1

How are study results presented?

Answer 1

1. Mean values (or mean changes from baseline)
2. Proportion (percentages, risk ratio, and odds ratio)
3. Time to an outcome (survival curve, hazard ratio)

Question 2

What types of outcomes can be successfully represented by means?

Answer 2

Continuous outcomes (ex. BP)

Question 3

What are proportions?

Answer 3

The fraction of the total that possesses the outcome

Proportions are often used to compare outcomes between groups

ex. Compare failure rates between groups of the Vitamin D study

Question 4

What are some types of proportions used to compare outcomes of studies?

Answer 4

1. Absolute Risk Reduction (ARR)
2. Relative Risk (RR)
3. Relative Risk Reduction (RRR)
4. Odds Ratio (OR)
5. NNT/NNH

Question 5

What is absolute risk reduction (ARR)?

Answer 5

The absolute difference between the probability of the event in the control group and probability of the event in the intervention group

ex. ARR of stroke= 8.3% (placebo) - 5.7% (Drug X) = 2.6%

Drug X reduces stroke by 2.6% vs. placebo

Question 6

What is relative risk (RR)?

Answer 6

This shows the “risk” of the outcome in the intervention group is compared to the “risk” in the control groups

ex. RR= Intervention risk/placebo risk = 5.7%/8.3% = 0.68

Question 7

Can continuous outcomes be used to determine relative risk (RR)?

Answer 7

No, relative risk can only be calculated for dichotomous outcomes (yes or no?)

Question 8

How are relative risk (RR) values analyzed?

Answer 8

RR = 1.0 (No difference in risk between groups)

RR < 1.0 (Less risk of outcome in intervention group

RR > 1.0 (Higher risk of outcome in intervention group)

Question 9

What is relative risk reduction (RRR)?

Answer 9

The degree to which baseline risk is reduced (or increased) by the intervention

RRR(relative risk reduction) = 1-RR(relative risk) = 1- 0.68 = 0.32

Drug X reduces the risk of stroke by 32% vs. placebo

Question 10

Can continuous outcomes be used to determine relative risk reduction (RRR)?

Answer 10

No, relative risk reduction (RRR) can only be calculated for dichotomous outcomes

Question 11

What is odds ratio (OR)?

Answer 11

This value shows the odds of the outcome occurring in the intervention group compared to the control group

Odds = (risk of having event)/(risk of not having event)

Odds Ratio = odds (intervention)/odds (placebo)

Question 12

How can odds ratio (OR) values be interpreted?

Answer 12

OR = 1.0 (No difference in odds between groups)

OR < 1.0 (Less odds of outcome in intervention group)

OR > 1.0 (Higher odds of outcome in intervention group)

Question 13

What is Number needed to treat (NNT)?

Answer 13

Number of subjects who would have to be treated (receive the intervention) in order for one additional subject to “benefit” in comparison to the control

Duration of study is also important when interpreting results

ex. NNT = 100%/ARD (absolute risk difference) = 100/2.6 = 38.5= 39

We need to treat 39 patients with Drug X to prevent stroke in one additional person compared to placebo

Question 14

What is Number needed to harm (NNH)?

Answer 14

Similar concept as NNT, except in NNH the outcome that we prevent is an adverse effect

ex. NNH = 100, Need to treat 100 people to see one additional person to not experience a defined side effect vs. the comparator group

Question 15

Is a small NNT always an indication for the studied therapy to be highly reccomended?

Answer 15

No, we need to look at the results of the study.

A study may have a small NNT, but both interventions studied may have grave adverse effects

Question 16

What are some ways study results can be compared to each other as time to outcome?

Answer 16

1. Survival curve
2. Hazard ratio

Question 17

What is the Kaplan-Meier survival curve?

Answer 17

Compares how long it takes subjects to reach the outcome (ex. median time until stroke)

Often reported as median survival time (time for half of the subjects to reach outcome)

Question 18

What is the survival (outcome survival) rate at the start of the study?

Answer 18

100%

Question 19

What is the hazard ratio (HR)?

Answer 19

This helps compare survival curves between groups

HR = 1.0 (No difference in hazard betweeen groups)

HR less than 1.0: Less hazard of outcome in intervention group

HR more than 1.0: Higher hazard of outcome in intervention group

Question 20

What are confidence intervals (CI)?

Answer 20

• Range where the true effect of the intervention (treatment) lies
• The narrower the CI, the more precise the results

ex. 0.32 (95% CI 0.22 to 0.40). If we run our study 100 times, in 95 trials our study will acheive results within the confidence interval

Question 21

What are advantages of confidence intervals?

Answer 21

CI can show us the magnitude of the effect, while p-values can only give us a binary outcome (significant, and insignificant)

CI values also show us the direction of effect

CI values also help distinguish between clinical and statistical significance

Question 22

How can confidence intervals be used to compare groups?

Answer 22

If the CI crosses the threshold for “no difference”, then the result is not statistically significant

For mean values or proportions: no difference = 0

For relative risk or odds ratio: no difference = 1

review slides 33 and 34

Question 23

What are superiority studies?

Answer 23

Most studies are superiority studies

These studies want to show an intervention is better than control (active/placebo)

Question 24

What are non-inferiority trials?

Answer 24

These studies try to determine whether new intervention is not substantialy worse than an established intervention

What is considered to be not substantially worse must be predetermined before conducting study

Question 25

Why are intention-to-treat (ITT) and per-protocol analyses performed for non-inferiority trials?

Answer 25

IIT (include all participants, including droupouts. Therefore minimizing effects of drug in results)

per-protocol (only those that followed treatment fully, but will see greater drug effects)

Question 26

What are equivalence studies?

Answer 26

Performed rarely

The new intervention is neither worse nor better than an established intervention

Question 27

Review slide 42 for review of interpreting non-inferiority margin

Question 28

What are propensity scores?

Answer 28

They are used in observational studies to determine the probability that a subject would be in a particular treatment group based on their observed baseline characteristics

Question 29

What is adaptive clinical trial design?

Answer 29

These types of studies have the ability to have pre-determined modifications to the study design (this makes them these types of studies more efficient compared to traditional trials)