- Students often have difficulty grasping the  difference between random sample and random assignment. This is completely understandable  because both words have the word random in it. However, understanding the difference  between the two concepts is essential. This video will address exactly this  issue. By the end of this video, you should be able to distinguish the difference  between a random sample and a random assignment. Say we want to run an experiment to see  whether a treatment we're interested in has some effect. As an example, let's say  that a politician wants to publish an ad, and we want to know whether the ad  will increase support among voters for that politician. The first step is  defining the population. In this case, the population would be all voters. The second step  is drawing a random sample from the population. Drawing a random sample assures that we have a  sample that is representative of the population. Remember that this allows us to make inferences  about the population from our sample. Third, to find out if a treatment has an effect, we want to randomly assign some units to the  control group and some units that the treatment group. The module on experiments explains  why randomizing the treatment is desired. But just to recap, it's because, as can  be seen here, it creates a treatment and a control group that are  very similar to each other. If the treatment has any effect, we  can be fairly sure that it's due to the treatment itself and not because the  treatment and control groups are different. Once we have our treatment and control groups, we want to give the treatment group the  treatment, in this case show them the ad, and not give the control group the treatment, so  not show them the ad. After this has been done, we want to measure support for this politician  in both the treatment and the control group. When we compare the outcomes of the treatment  and control groups, if we find any difference in support for this politician we can be  fairly confident that the ad had this effect. So notice that a random sample are the  units that we want to collect data on while random assignment decides which of  these units get the treatment and which don't.