The term iniciales sesgadas means the initials are NOT randomly assigned to the sample but the initials correspond to the patient, thus the researcher can match the sample to the research subject therefore introducing an element of bias in the selection

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Note added at 1 hr (2018-03-15 11:36:35 GMT)
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The random assignment of subjects to one or another of two groups (differing only by the intervention to be studied) is the basis for measuring the marginal difference between these groups in the relevant outcome. Randomisation should equally distribute any confounding variables between the two groups, although it is important to be aware that differences in confounding variables may arise through chance.

Randomisation is one of the cornerstones of the RCT7 and a true random allocation procedure should be used. It is also essential that treatment allocations are concealed from the investigator until recruitment is irrevocable, so that bias (intentional or otherwise) cannot be introduced at the stage of assigning subjects to their groups.8 The production of computer generated sets of random allocations, by a research support unit (who will not be performing data collection) in advance of the start of the study, which are then sealed in consecutively numbered opaque envelopes, is an appropriate method of randomisation. Once the patient has given consent to be included in the trial, he/she is then irreversibly randomised by opening the next sealed envelope containing his/her assignment. (mj.bmj.com/content/20/2/164)

I think Liz, that biased initials in this context is a term applied to the samples before they were randomized, at which time they would be randomly assigned initials again.