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Unconfounded comparison
A comparison between two  treatment groups that will give an unbiased estimate of the effect of treatment due to the study design. For a comparison to be unconfounded, the two treatment groups must be treated identically, apart from the randomised treatment. For instance, to estimate the effect of heparin in acute stroke, a trial of heparin alone versus placebo would provide an unconfounded comparison. However, a trial of heparin alone versus aspirin alone provides a confounded comparison of the effect of heparin.

Uncontrolled trial
A clinical trial that has no control group.

Unit of allocation
The unit that is assigned to the alternative interventions being investigated in a trial. Most commonly, the unit will be an individual person but, in a cluster randomised trial, groups of people will be assigned together to one or the other of the interventions. In some other trials, different parts of a person (such as the left or right eye) might be assigned to receive different interventions. See also unit of analysis error.

Unit of analysis error
An error made in statistical analysis when the analysis does not take account of the unit of allocation. In some studies, the unit of allocation is not a person, but is instead a group of people, or parts of a person, such as eyes or teeth.  Sometimes the data from these studies are analysed as if people had been allocated individually. Using individuals as the unit of analysis when groups of people are allocated can result in overly narrow confidence intervals. In meta-analysis, it can result in studies receiving more weight than is appropriate.