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The individuals in an experiment
The explanatory variables in an experiment
- is any specific experimental condition applied to the subjects. If an experiment has several factors, a treatment is a combination of specific values of each factor.
- If the experiment involves giving two different doses of a drug, we say that we are testing two levels of the factor.
- A response to a treatment.
- if it is larger than you would expect
- by chance (due to random variation among the subjects).
3 Principles of experimental design
- 1. control
- 2. Randomize
- 3. Replicate
the effects of lurking variables on the response, most simply by comparing two or more treatments.
use impersonal chance to assign subjects to treatments
use enough subjects in each group to reduce chance variation in the results.
is a situation in which no treatment is administered. It serves as a reference mark for an actual treatment (e.g., a group of subjects does not receive any drug or pill of any kind).
is a fake treatment, such as a sugar pill. It is used to test the hypothesis that the response to the treatment is due to the actual treatment and not to how the subject is being taken care of.
Getting rid of sampling biases
The best way to exclude biases in an experiment is to randomize the design. Both the individuals and treatments are assigned randomly.
A double-blind experiment is one in which neither the subjects nor the experimenter know which individuals got which treatment until the experiment is completed.
- Another way to make sure your conclusions are robust is to replicate your experiment—do it over. Replication ensures that particular results are not due to uncontrolled factors or
- errors of manipulation.
- is one in which neither the subjects nor the experimenter
- know which individuals got which treatment until the experiment is completed.
Completely randomized designs
- individuals are randomly assigned to
- groups, then the groups are randomly assigned to treatments.
Matched pairs designs
- Choose pairs of subjects that are closely matched— e.g., same sex, height, weight, age,
- and race. Within each pair, randomly assign who will receive which treatment.
- It is also possible to just use a single person and give the two treatments to this person over time in random order. In this case, the “matched pair” is just the same person at
- different points in time
subjects are divided into groups, or blocks, prior to the experiment to test hypotheses about differences between the groups.