Random Assignment
To ensure that there are no unexpected differences between the experimental and control groups, good experimental design uses random assignment to determine which participants are in each group. This helps to minimize the impact of other variables that the researcher has no control over. Do not confuse this with random sampling! (In fact, most experiments do not use random sampling to find their sample.)
In our previous fish experiment, random assignment means that no matter how the 2,000 fish eggs were obtained, the ecologist would use some system of randomization to divide the eggs into two equal groups. For example, the researcher can use a random number generator to select 1,000 fish to be in the control group and then assign the remaining 1,000 fish to the experimental group. This would help to minimize the impact of other potential variables, such as the genetics and health of individual eggs.
Video Placement
[Perspective Video with a worked example: a 3-instructor video that explains the difference between random sampling and random assignment. These types of similar terms present a significant challenge to students who don’t yet have a mental landscape of statistics. This explanation should be followed by one or two examples in the style of Question 4]
Now it’s your turn to try identifying random assignment using statistical descriptions.
question 4
Identify the scenario(s) below that describe(s) random assignment (not random sampling). There may be more than one correct answer.
- a) A political pollster uses an electronic database to randomly generate a list of telephone numbers to call with questions about an upcoming election.
- b) A researcher uses a random number generator to determine which patients will receive an experimental medication and which patients will receive standard treatment.
- c) A researcher flips a coin to decide whether to assign a new participant to the experimental or control group.
- d) A professor uses a random number generator to decide which students to contact for a course evaluation.
