Help with this research methods question?

Random assignment means that you have not pre-selected a population or grouping for certain characteristics. Random selection means that every individual organism or object in your test has an equal chance of being selected.

For example if you take a poll in a town and only ask your questions only to people under the age of 25, you run the risk of having a more biased outcome whereas if you ask your questions to people of every age, you are going to get a wider and thus more significant set of data as answers from people age 65 may be different from those who are age 40 or age 8.

The same works for scientific experiments. The more random your selections are from your target group, the more likely that your results will be valid.

If you are trying to see whether a particular vaccine is effective against a disease, you have to try it against cells from all different groupings of people (the more the better). People with certain gene structure may react differently to same vaccine, and a random sampling can help determine this, if your sample size is large enough.