![]() These twins represent an enormous resource in the study of the brain, and since their condition is very rare, it is likely that as long as their family agrees, scientists will follow these girls very closely throughout their lives to gain as much information as possible. There is evidence that the two girls are connected in a part of the brain called the thalamus, which is a major sensory relay center. These particular twins are unique because Krista and Tatiana are conjoined twins, connected at the head. In 2011, the New York Times published a feature story on Krista and Tatiana Hogan, Canadian twin girls. There is statistical software that will randomly assign each of the fourth graders in the sample to either the experimental or the control group. With random assignment, all participants have an equal chance of being assigned to either group. Now that we have a sample, the next step of the experimental process is to split the participants into experimental and control groups through random assignment. With a representative group, we can generalize our findings to the larger population without fear of our sample being biased in some way. In summary, because we cannot test all of the fourth graders in a city, we want to find a group of about 200 that reflects the composition of that city. With this more manageable population, we can work with the local schools in selecting a random sample of around 200 fourth graders who we want to participate in our experiment. We should include students from various income brackets, family situations, races, ethnicities, religions, and geographic areas of town. But all fourth graders is a very large population, so we need to be more specific instead we might say our population of interest is all fourth graders in a particular city. In our example, let's say we decide our population of interest is fourth graders. This means that the percentages of characteristics in the sample-sex, ethnicity, socioeconomic level, and any other characteristics that might affect the results-are close to those percentages in the larger population. Random samples are preferred because if the sample is large enough we can be reasonably sure that the participating individuals are representative of the larger population. ![]() A random sample is a subset of a larger population in which every member of the population has an equal chance of being selected. ![]() Dependent variable - not manipulated/influenced. Independent variable - manipulated/influenced. The independent variable is influenced and manipulated during the experiment. The dependent variable is measured to see how much effect the independent variable had.
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