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Task 19: How are experiments designed?

Some statistical concepts and designing an experiment

What is a random sample?


Definition: A random sample is a subset of individuals that are randomly selected from a population. Because researchers usually cannot obtain data from every single person in a group, a smaller portion is randomly selected to represent the entire group as a whole. The goal is to obtain a sample that is representative of the larger population...

Extraneous and Confounding Variables and Systematic vs Non-Systematic Error Extraneous Variables


are undesirable variables that influence the relationship between the variables that an experimenter is examining. Another way to think of this, is that these are variables the influence the outcome of an experiment, though they are not the variables that are actually of interest. These variables are undesirable because they add error to an experiment. A major goal in research design is to decrease or control the influence of extraneous variables as much as possible. For example, let’s say that an educational psychologist has developed a new learning strategy and is interested in examining the effectiveness of this strategy. The experimenter randomly assigns students to two groups. All of the students study text materials on a biology topic for thirty minutes. One group uses the new strategy and the other uses a strategy of their choice. Then all students complete a test over the materials. One obvious confounding variable in this case would be pre-knowledge of the biology topic that was studied. This variable will most likely influence student scores, regardless of which strategy they use. Because of this extraneous variable (and surely others) there will be some spread within each of the groups...

Dependent Variable:


A dependent variable is what you measure in the experiment and what is affected during the experiment. The dependent variable responds to the independent variable. It is called dependent because it "depends" on the independent variable. In a scientific experiment, you cannot have a dependent variable without an independent variable. Example: You are interested in how stress affects heart rate in humans. Your independent variable would be the stress and the dependent variable would be the heart rate. You can directly manipulate stress levels in your human subjects and measure how those stress levels change heart rate.

If you have decided that an experiment is the best approach to testing your hypothesis, then you need to design the experiment. Experimental design refers to how participants are allocated to the different conditions (or IV groups) in an experiment. Once the sample has been selected the researcher needs to make a decision about how their behavior will be recorded. In an experiment...

... 3. Matched Pairs:

See the complete article here:

One pair must be randomly assigned to the experimental group and the other to the control group. Matched Pairs Experimental Design Pro: Reduces participant (i.e. extraneous) variables because the researcher has tired to pair up the participants so that each condition has people with similar abilities and characteristics. Pro: Avoids order effects, and so counterbalancing is not necessary. Con: Very time-consuming trying to find closely matched pairs. Con: Impossible to match people exactly, unless identical twins! ...

Okay now answer the following questions:

What is an extraneous variable?
and what is a dependent variable?

Task Discussion