Experimental event rate

In epidemiology and biostatistics, the experimental event rate (EER) is a measure of how often a particular statistical event (such as response to a drug, adverse event or death) occurs within the experimental group (non-control group) of an experiment.[1]

This value is very useful in determining the therapeutic benefit or risk to patients in experimental groups, in comparison to patients in placebo or traditionally treated control groups.

Three statistical terms rely on EER for their calculation: absolute risk reduction, relative risk reduction and number needed to treat.

Worked example

In a trial of hypothetical drug "X" where we are measuring event "Z", we have two groups. Our control group (25 people) is given a placebo, and the experimental group (25 people) is given drug "X".

Event "Z" in control group : 4 in 25 people Control event rate : 4/25

Event "Z" in experimental group : 12 in 25 people Experimental event rate : 12/25

Another worked example is as follows:

Example 1: risk reduction Example 2: risk increase
Experimental group (E) Control group (C) Total (E) (C) Total
Events (E) EE = 15 CE = 100 115 EE = 75 CE = 100 175
Non-events (N) EN = 135 CN = 150 285 EN = 75 CN = 150 225
Total subjects (S) ES = EE + EN = 150 CS = CE + CN = 250 400 ES = 150 CS = 250 400
Event rate (ER) EER = EE / ES = 0.1, or 10% CER = CE / CS = 0.4, or 40% EER = 0.5 (50%) CER = 0.4 (40%)
Equation Variable Abbr. Example 1 Example 2
EER CER < 0: absolute risk reduction ARR ()0.3, or ()30% N/A
> 0: absolute risk increase ARI N/A 0.1, or 10%
(EER CER) / CER < 0: relative risk reduction RRR ()0.75, or ()75% N/A
> 0: relative risk increase RRI N/A 0.25, or 25%
1 / (EER CER) < 0: number needed to treat NNT ()3.33 N/A
> 0: number needed to harm NNH N/A 10
EER / CER relative risk RR 0.25 1.25
(EE / EN) / (CE / CN) odds ratio OR 0.167 1.5
EER CER attributable risk AR ()0.30, or ()30% 0.1, or 10%
(RR 1) / RR attributable risk percent ARP N/A 20%
1 RR (or 1 OR) preventive fraction PF 0.75, or 75% N/A

See also

References