**Core Concept**
Randomization is a statistical technique used in clinical trials to minimize bias and ensure that the groups being compared are similar in terms of known and unknown confounding variables. This is achieved by allocating participants to treatment or control groups through a random process, such as a random number generator.

**Why the Correct Answer is Right**
The major purpose of randomization is to reduce selection bias and confounding variables. By randomly assigning participants to treatment or control groups, randomization ensures that any differences between the groups are due to the intervention being tested, rather than pre-existing differences between the groups. This helps to establish causality and increases the validity of the trial results. Randomization also helps to prevent allocation bias, where participants are allocated to groups based on characteristics that are not relevant to the outcome being measured.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because randomization is not primarily used to reduce variability between groups. While randomization does help to minimize variability, this is not its primary purpose.
**Option B:** This option is incorrect because randomization is not primarily used to increase the sample size of a clinical trial. While randomization can help to increase the sample size by reducing bias, this is not its primary purpose.
**Option C:** This option is incorrect because randomization is not primarily used to measure the effectiveness of a treatment. While randomization is used to measure the effectiveness of a treatment, this is not its primary purpose.

**Clinical Pearl / High-Yield Fact**
A key benefit of randomization is that it allows clinicians to make inferences about the population from which the sample was drawn, as long as the sample is representative of the population.

**Correct Answer:** D. Randomization helps to prevent confounding variables and selection bias, thereby increasing the validity of the trial results.