Critical evaluation: Learning exercises

Last updated: Tuesday, July 14, 2015

Health Education Wessex and the Southampton Medicines Advice Service have produced e-learning on critical appraisal of clinical trials. It reinforces and builds upon a lot of the material in the tutorial you have just completed.

Allow about 45 minutes to complete the e-learning.

Example critical evaluation exercises

In practice, we should not rely on abstracts from clinical trials to form judgements about evidence, but for reasons of space we will look at edited abstracts from two published trials here.

Read through each of the abstracts below and then answer the questions.

1.  Extended-duration venous thromboembolism prophylaxis in acutely ill medical patients with recently reduced mobility. A randomised trial. (based on Hull et al. Ann Intern Med 2010;153:8-18)

Objective: To evaluate the efficacy and safety of extended-duration enoxaparin thromboprophylaxis in acutely ill medical patients.
Design: Randomised, parallel, placebo-controlled trial. Randomisation was computer-generated. Allocation was centralised. Patients, caregivers, and outcome assessors were blinded to group assignment.
Patients and intervention: Acutely ill medical patients 40 years or older with recently reduced mobility were given subcutaneous enoxaparin, 40mg/day (2975 patients), or placebo (2988 patients), for 28 ± 4 days after receiving open-label enoxaparin for an initial 10 ± 4 days.
Measurements: Incidence of venous thromboembolism (VTE) up to day 28 and of major bleeding events up to 48 hours after the last study treatment dose.
Results: Extended-duration enoxaparin reduced VTE incidence compared with placebo (2.5% vs. 4%, respectively). Major bleeding events occurred in 0.8% of the enoxaparin and 0.3% of the placebo groups.
a) What is the implication of allocation being 'centralised'?
b) What are the absolute risks of VTE in the enoxaparin and placebo groups?
c) What are the absolute risk reduction and relative risk reduction? What do these numbers tell us?
d) What are your initial conclusions about the results of this trial?
e) What other information, not included in the abstract, would you need to know to be able to appraise this paper?
When you have attempted these questions, then look at the suggested answers we have prepared for you. Make sure you have tried to answer the questions yourself first, though!

2. Does single application of topical chloramphenicol to high-risk sutured wounds reduce incidence of wound infection after minor surgery? Prospective randomised placebo-controlled double-blind trial. (based on Heal et al. Br Med J 2009;338:a2812) 

Objective: To determine the effectiveness of a single application of topical chloramphenicol ointment in preventing wound infection after minor dermatological surgery.
Design: Prospective randomised placebo-controlled double-blind multicentre trial in primary care in Queensland, Australia.
Intervention: 488 patients were treated with a single topical dose of chloramphenicol and 484 with a placebo consisting of paraffin ointment.
Results: The incidence of infection in the chloramphenicol group was 6.6% and 11.0% in the placebo group, P=0.01.
a) What are the absolute risks of infection in the chloramphenicol and placebo groups?
b) What are the absolute risk reduction and relative risk reduction?
c) Calculate the NNT.
d) What can you conclude from these results?
e) What other information, not in the abstract, would you need to help you decide if the results of the paper could be generalised to patients having minor dermatological surgery in a UK general practice?
We have also prepared some suggested answers to these questions. Please look at them when you've attempted the questions yourself.

Finally, if you have access to the MiCAL training package (subscription required) you may like to study the critical evaluation module that it contains.