# Stats in EBM Flashcards

1
Q

What is incidence?

A

A rate measuring the number of new cases of a particular disease arising in a population at a certain time period (the tap in the bath analogy)

2
Q

What is prevalence?

A

A ratio of all cases of disease existing in a given population at a given time (the bath in the bath analogy)

3
Q

What is risk ratio?

A

Ratio of incidence rate in group A vs incidence rate in group B

eg 25/100 get disease in A, 10/100 people get disease in B
Risk ratio = 0.25/0.01 = 2.5x more risk of death in group A

4
Q

What is odds ratio?

A

Odds of outcome in exposed group vs odds of outcome in unexposed group
Eg:
In group A 25/100 got disease so thats 75 that didn’t. So 25:75 = 1:3
In group B 10/100 got disease so thats 10:90 or 1:9

Odds ratio = 1:3 / 1:9 = 3
(because 1/3 x 9/3 = 9/3 = 3)

5
Q

What is the absolute risk?

A

Risk of acquiring given disease over given period of time eg 35/100 = 35%

6
Q

What is absolute risk difference?

A

Difference between each risk - eg if group A 25% got disease and group B 10% did, risk difference is 15%

7
Q

Why is risk difference useful?

A

Can express as relative risk so eg 25/100 vs 10/100
Group A is 2.5x more likely to get the disease - ‘more than doubles your risk’

Can seem more scary

8
Q

What is P value?

A

Measure of statistical significance - probability that the effect could have occured due to change

Therefore p value less than or equal to 0.05 is enough to reject the null and be statistically significant

9
Q

Problems with P values

(3)

A

Can never rule out chance

Gives no indication of size of the effect which is statistically significant

P value gives **no idea of the range of uncertainty **- this is what confidence intervals are for

10
Q

What is a confidence interval?

A

Gives range of uncertainty around the estimate you have found

11
Q

When is the confidence interval statistically significant?

A

If it does NOT contain 1 - it is significant

If it does not contain 1 p value will always be less than or equal to 0.05

12
Q

What is the line/point of no effect in confidence intervals for relative risk ratio?

A

1 is the point of no effect - if ratio is 1 this means outcomes are same

13
Q

What is the ‘mean absolute difference’? aka MAD

A

Comparison between two outcomes

Eg if group A had weight loss of 1kg
Group B had weight loss of 0.9kg
Absolute difference is 0.1kg

14
Q

What would be the line of no significance (eg what would the confidence intervals have to cross) for a MAD to be non-significant?

A

0 - if the CI cross 0 this shows there is no difference between some of the two groups

15
Q

How to interpret Forrest Plot?

A
• Dot/square is odds ratio usually
• Lines extending out are confidence interval
• Line down centre is line of no significance (eg 1 if ratio, 0 if MAD)
• Diamond at base is sum of all data and width is confidence interval
16
Q

What was form as a result of needing systematic reviews to analyse different studies and pool evidence?

A

Cochrane Collaboration