This page contains a detailed example from Visual Rx followed by links to external journal articles that use or reference Visual Rx.
There are also numerous examples of Cates plots in articles on my web site which are all collated here.
Detailed Example
The Cates Plots shown below are examples of the output taken from Visual Rx (version 4). The data is taken from the updated review “Antibiotics for acute otitis media in children” on the Cochrane Library in June 2015. I think they provide a good picture of the benefits and harms which can be understood by patients and their health care professionals. In this case the Cates plots show that, for every 100 treated with antibiotics, five are freed from pain at 2-3 days as opposed to seven children who develop diarrhoea, vomiting or rash . If you read on you will see how this works.
The Cates plot in Figure 1 represents 100 children who are all given antibiotics for ear infections. The 84 green faces are children who would have been free from pain at 2 to 3 days even if they had not received an antibiotic. The 11 red faces are children who are still in pain even with antibiotics. The 5 yellow faces are the only children who show a benefit; they would have been in pain without the antibiotic but are not when they receive one. Since it is not possible to identify which children will benefit all 100 need to be given the antibiotic for 5 to benefit.
Figure 1: Cates plot of pain at 2-3 days in children given antibiotics versus placebo for acute otitis media
The calculated risk on antibiotics was 11.13% (95% CI 9.06% to 13.67%) compared to 15.9% on placebo. This calculation is carried out by using the pooled relative risk from the Cochrane review (RR 0.70: 95% CI 0.57 to 0.86) and applying this to the 15.9% risk on placebo.
The resulting risk difference is 4.77 percentage points and then the NNT(B) is calculated by taking the inverse of this risk difference (100/4.77 = 21). The confidence interval for the NNT is calculated in the same way using the placebo risk and applying the lower and upper confidence limit of the relative risk. The 95% CI for the NNT(B) ranges from 15 to 45. The results are generated in a table by Visual Rx as shown below.
Table 1. Table of Natural frequencies for pain
| Antibiotics for acute otitis media in children | ||
|---|---|---|
| Outcome: pain | ||
| Duration: 2-3 days | ||
| Control group risk | Treatment group risk (95% CI) | NNTB (95% CI) |
| 15.90% | 11.13% | 21 |
| (9.06% to 13.67%) | (NNTB 15 to NNTB 45) | |
| In the control group 16 people out of 100 had pain over 2-3 days, compared to 11 (95% CI 9 to 14) out of 100 for the active treatment group. | ||
In contrast the next Cates Plot in Figure 2 shows that when all 100 children are given antibiotics an extra 7 will suffer diarrhoea, vomiting or a rash.
Figure 2: Cates plot of diarrhoea, vomiting or rash over the study period in children given antibiotics versus placebo for acute otitis media
In this case the control risk and calculated risks on antibiotics (with NNTH and its confidence interval) are generated by Visual Rx (version 4) as shown below, and this shows that the number needed to cause a harmful outcome in one child (NNH) is 14.
Table 2. Table of Natural frequencies for diarrhoea, vomiting or rash
| Antibiotics for acute otitis media in children | ||
|---|---|---|
| Outcome: diarrhoea, vomiting or rash | ||
| Duration: the duration of the trial | ||
| Control group risk | Treatment group risk (95% CI) | NNTH (95% CI) |
| 19.60% | 27.05% | 14 |
| (23.32% to 31.16%) | (NNTH 9 to NNTH 27) | |
| In the control group 20 people out of 100 had diarrhoea, vomiting or rash over the duration of the trial, compared to 27 (95% CI 23 to 31) out of 100 for the active treatment group. | ||
Journal Articles
Below is a further list of some external resources that use, or discuss, the smiley face plots produced by Visual Rx.
Understanding Uncertainty — Spiegelhalter 6 (3): 196 — Annals of Family Medicine
The Legacy Website of the National Prescribing Centre also contains a collection of Patient Decision Aids that feature Cates plots prominently.