The deliberate planning of a study in which the researcher imposes treatments on subjects and measures responses, using control groups, randomization, replication, and (where possible) blinding to establish cause-and-effect relationships.
You want to know if a fertilizer helps plants grow. You can't just give it to some plants and hope for the best—you need a plan: a group that gets the fertilizer, a group that doesn't (control), random assignment so the groups are fair, enough plants so one weird result doesn't fool you (replication), and ideally the person measuring growth doesn't know which group is which (blinding).
Showing a random 20 of 50 problems.
Example 1
medium
Why is using volunteers (e.g., self-selected subjects) generally a weak experimental design?
Example 2
medium
Subjects are split into age blocks (under-40, over-40), then randomly assigned to treatment or control within each block. What design is this?
Example 3
medium
A trial compares 4 medications across 200 patients, with a balanced CRD. How many per medication? How can the researcher randomly assign them using only a coin?
Example 4
easy
A teacher tests two teaching methods. She teaches Group A (morning) with Method 1 and Group B (afternoon) with Method 2. Identify the confounding variable and explain how to fix the design.
Example 5
easy
A drug trial gives one group the real pill and another an identical sugar pill. What is the sugar pill called?
Example 6
hard
A trial randomly assigns 50 patients to drug, 50 to placebo. The drug group improves 60%, placebo 40%. Why is this NOT proof the drug caused improvement without further analysis?
Example 7
medium
Design an experiment to test whether caffeine improves test performance. Identify: explanatory variable, response variable, control group, treatment group, and how to control for confounders.
Example 8
hard
A study wants to test three fertilizer types (A, B, C) on crop yield across 12 fields. Design a completely randomized design (CRD) and a randomized block design (RBD) if the 12 fields have 4 different soil quality levels.
Example 9
challenge
A school wants to test if a new lunch menu improves attendance. Two schools are picked: School A switches menus; School B does not. Why is this NOT a true experiment, and how would you fix it?
Example 10
medium
True or false: a well-designed experiment with random assignment CAN establish cause-and-effect.
Example 11
easy
True or false: an experiment without a control group can still clearly establish that the treatment caused the effect.
Example 12
medium
Fill in the blank: a ____ design assigns subjects to blocks based on a relevant characteristic, then randomizes within each block.
Example 13
easy
What is a confounding variable?
Example 14
hard
In a matched-pairs design, two subjects with similar characteristics are paired, and within each pair one is randomly assigned to treatment, the other to control. How does this differ from a CRD?
Example 15
medium
In an experiment with 120 subjects and 3 treatments (A, B, C), how many should go to each group in a balanced completely randomized design?
Example 16
easy
What design feature uses chance to assign subjects to treatment groups?
Example 17
easy
A researcher does NOT know which subjects got the real drug; subjects don't know either. What is this called?
Example 18
medium
A factor in an experiment has 3 levels (low, medium, high dose). Including a placebo as a 4th level, what is the new total number of groups?
Example 19
easy
In an experiment, what is the group that receives the actual treatment called?
Example 20
easy
Fill in the blank: ____ assignment uses chance (e.g., a coin flip) to put subjects in groups.