Study Designs By Az and Omar

This aim of this session is to make sure you understand - high level – what each type of study is. The slides will be useful to go over in the run up to your exams. Pay more attention to us for today though. If you are going through this just before your exams then…. Good Luck!!!

Everything you need to know in a nutshell

Hierarchy of evidence Rigour Systematic reviews - meta-analyses - non meta-analytic reviews Experimental studies - Randomised controlled trials - Controlled trials Observational studies - Cohort studies - Case-controlled studies Descriptive Studies - cross sectional designs Rigour

What is a systematic review? It is a way of combining trials using a pre-defined, explicit methodology which includes steps to minimise bias

What is a Meta Analysis? The name given to a combination of trials which results in the production of an overall statistic

Our attempt at an explanation… Trial 1 (N=1,000) obtained a result of ‘4’ (±CI) Trial 2 (N=1,000) obtained a result of ‘5’ (±CI) We want to be able to say “It’s like a single trial (N=2,000) obtained a result of 4.5 (±CI) This would be a systematic review with a meta analysis If the two trials were so different that we can’t combine them, it is still a systematic review... Just without a meta analysis

Forest Plot of the results

Funnel Plots Funnel plots are a way of assessing whether the results of a review have been influenced by ‘publication bias’ If the plot is symmetric, like an inverted V, this is interpreted as demonstrated that there is probably no publication bias If the plot is asymmetric, the interpretation is that publication bias is likely

Symmetrical funnel plot As the studies become less precise (i.e. higher standard error), you would expect the results (given here as a log odds ratio) of the studies to be more variable, scattered to both sides of the more precise larger studies. When you plot your studies onto a funnel plot, you may find it is not symmetrical and does not resemble an inverted funnel. This may be due to publication bias, however there are other factors leading to an asymmetrical plot.

Asymmetrical funnel plot Shows publication bias!!! Findings from a review of Aversive Smoking for smoking cessation. The outcome is risk of quitting, so the larger the OR the better aversive smoking works.

What is a Randomised Control Trial? A study in which people are allocated at random (by chance alone) to receive one of several clinical interventions. One of these interventions is the standard of comparison or control. (The control may be a standard practice, a placebo, or no intervention at all.)

What is a Case Control Study? Case control studies involve comparing subjects with a condition (the cases) to subjects without the condition (the controls). The level of exposure to a factor or factors is determined for both groups and compared. If the prevalence of exposure is higher in cases than in controls then the exposure might be a risk factor. The point is – we take a group of people today who already have the disease, and a group who don’t – and we LOOK BACK at their lives to try to figure out why

What is a Cohort Study? A cohort is a group of people who have something in common A cohort study is one in which a group of people are followed up over time So the point is – We know what happened in these people’s pasts. We then wait to see if/how it effects them. You will have been introduced to cohort studies in Margaret Thorogoods lecture on case control studies – so you will know something about them already I think a cohort was a group of ten Roman soldiers – a group who stuck together over time – well until they got themselves killed You are a cohort

What is a Cross Sectional Study?

The following are questions that are taken from the peer support presentation that was given to us last year. To give credit where it’s due – they were written by two mambers of the current third year called Amritpal and Joshua.

Define clinical trial Any form of planned experiment which involves patients and is designed to elucidate the most appropriate method of treatment of future patients with a given medical condition

Define randomisation Random allocation of participants into study groups wherein each participant has an equal chance of being put into each group, analogous to flipping a coin

What are the advantages of random allocation? [4] Gives equal chance of receiving each treatment In long run leads to groups that are likely to be similar in characteristics by chance Reduces selection bias if patients enter trial before randomisation Is used in other experimental settings

What is the placebo effect? The psychological benefit gained from being treated, even if the treatment has no physiological effects

What is a placebo? An inert substance which is identical (in terms of appearance, taste and packaging) to the treatment being investigated

In what situation would a placebo not be used? When there is an existing standard treatment

Name and explain TWO ethical issues that you will consider when advising the patient about the RCT [2+2] Clinical equipoise - is there reasonable uncertainty about what is the best treatment? Scientifically robust - is the sample size big enough to gain a clinically important effect? Is the question being asked relevant? Informed consent - has it been taken? Written or verbal?

How can informed consent issues be overcome? [3] Explain what the different treatment groups are Explain allocation is random Explain the patient may withdraw at any time (verbally and written)

As treated analysis Intention-to-treat Bob has started a trial looking at the efficacy of vaseline on genital friction burns. He initially has 160 medical students equally split into the placebo and vaseline group. At the end of the trial 30 people have dropped out from the vaseline group and 23 from the placebo group. Bob only analyses those who complete the study. What kind of analysis is this? What kind of analysis is most clinically relevant? As treated analysis Intention-to-treat