In this spotlight video, we will discuss random sampling and random assignment,
two concepts that sound similar, but serve quite different purposes in study design.
Random sampling occurs when subjects are being selected for a study.
If subjects are selected randomly from the population,
then each subject in the population is equally likely to be selected, and
the resulting sample is likely representative of the population.
Therefore the study's results are generalizable to the population at large.
Random assignment occurs only in experimental settings,
where subjects are being assigned to various treatments.
Taking a close look at our sample, we usually see that the subjects exhibit
slightly different characteristics from one another.
Through a random assignment, we ensure that these different characteristics
are represented equally in the treatment and control groups.
This allows us to attribute any observed difference between the treatment and
control groups, to the treatment being observed on the subjects,
since otherwise these groups are essentially the same.
In other words,
random assignment allows us to make causal conclusions based on the study.
Let's give a quick example.
Suppose you want to conduct a study, evaluating whether people read serif fonts
or sans serif, or in other words, without serif fonts faster.
Note that serifs are this small jacketed pieces at the ends of each character.
Ideally, he would first randomly subjects for your study from your population.
Then, you assigned the subjects in your sample to two treatment groups.
One, where they read some text in serif font, and
the other where they read the same text in sans serif font.
Through random assignment, we ensure that other factors that may be contributing to
reading speed indicated here with the different colors or the subjects.
For example, fluency or how often the subject reads for
leisure, are represented equally in the two groups.
We call such variables confounders, or confounding variables.
In this setting, if we observe any difference between the average reading
speeds of the two groups, we can actually attribute it to the actual treatment,
the font type, and know that it's likely not due to a confounding variable.
So to recap, sampling happens first, and assignment happens second.
