Why does assigning trial participants by chance help compare treatments?

If people choose different treatments, the groups may already differ in age, health or other factors. A later difference in outcomes could reflect those starting differences rather than an effect of the treatment. Comparing results fairly therefore begins before any result is measured.

A randomized trial assigns eligible participants to comparison groups using a chance-based process. Researchers and participants do not simply choose the treatment they expect to suit each person best. This helps prevent, for example, healthier participants from being deliberately or unintentionally directed toward one treatment. The randomization method is part of the study's design.

Researchers then compare outcomes using a plan established for the study. Masking, where possible, means keeping participants or assessors unaware of which treatment was assigned, reducing the influence of expectations on behavior or measurement. The comparison can use standard care or another ethically appropriate control; it does not have to use a placebo.

Randomization balances characteristics in expectation, not perfectly in every actual sample. Sample size, missing observations, whether participants follow the assigned treatment, and chance variation still affect conclusions. A well-conducted trial supplies evidence about a specified intervention and population. Applying that evidence elsewhere requires attention to those boundaries.