what is the difference between random assignment and random selection?
You pull the subjects for a study from the population. If this process utilizes random sampling, you can generalize back to the population. If you have an experiment, you would then take your subjects and divide them into two groups–a control group (gets placebo or no treatment) and the experimental group (gets the independent variable). The division into the two groups should be done by random assignment so that you can control for confounding variables.
Random Assignment is done in an Experiment. When studying the effect of Advil vs a Placebo, one would randomly assign some to take the Advil and others to take a placebo. This reduces the chances for bias.
Random Selection is used in a survey. Of all the students in your school, you select the names of every fifth student on a class list to take part in a survey on the value of study groups.
Random selection refers to how the sample is drawn from the population as a whole, while random assignment refers to how the participants are then assigned to either the experimental or control groups. It is possible to have both random selection and random assignment in an experiment.