![]() a group that undergoes all experimental procedures except for the variable being investigated and which are specific to the experimental question being studied. A primary control is what we typically think of as a control, i.e. Both have a value but in different areas. There should also be a distinction between what we could call primary and secondary controls. Also, just like test treatments, they need to be subject to the same randomisation and blinding procedures and must be included in the experimental design and analysis. Positive controls are in a comparable position to your test treatment: they need a good negative control to be of any use. They might also be used to show the magnitude of effect that is possible with an active substance. They are used to show that the study can detect an effect in the desired direction and thus that the experimental protocol is sensitive to interventions expected to have an effect. In contrast, positive controls are expected to have an effect. These controls are necessary because all studies are open to unexpected effects. ![]() What are generally referred to as negative controls include vehicle and sham groups and are expected to show what happens in the absence of the intervention of interest. Note that in many studies these terms are used very loosely and, as Kramer and Font ( 2017) rightly point out, a description of what the control group is being used for is better than a label such as positive or negative which might be misleading. These can broadly be classed into negative and positive controls. Several types of control groups have been described in the literature, including positive, negative, sham, vehicle and comparative (Johnson and Besselsen 2002). A good control group allows you to do this – a bad control group means you cannot make valid comparisons to evaluate the activity of your test condition and, even worse, means you may end up drawing invalid conclusions. To unambiguously demonstrate that your experimental treatment has (or has not) had an effect in your test system, there needs to be a value against which you can compare it. The choice of appropriate control groups is intimately tied to the aims of your study. If anything, the opposite is true: the control group is a tacit acknowledgement, not only of all the things that we can’t control but of those things that we are not even aware of, the unknown unknowns. The term sounds comforting, as if we have managed to somehow rein in the experiment and submitted the study to our will. The purpose of the control group is to understand the influence of the third. Through good experimental design, we try to eliminate as much as possible the influence of the first two, the things we know about. ![]() KeywordsĪs Donald Rumsfeld famously said about weapons of mass destruction, there are known knowns, known unknowns and unknown unknowns. When used correctly, a good control group not only validates your experiment it provides the basis for evaluating the effect of your treatments. Historical and baseline control groups serve a slightly different role and cannot fully replace control groups run as an integral part of the experiment. It also means that in almost all cases, contemporaneous control groups are required. Because of this it is essential that they are treated as any other experimental group in terms of subjects, randomisation, blinding, etc. Perhaps most importantly they help you understand the influence of variables that you cannot fully eliminate from your experiment and thus include them in your analysis of treatment effects. Although they usually give results we can anticipate, they are an essential component of all experiments, both in vitro and in vivo, and fulfil a number of important roles in any experimental design. Control groups are expected to show what happens in the absence of the intervention of interest (negative control) or the effect of an intervention expected to have an effect (positive control).
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