The independent variable is defined as "experimental factor manipulated to determine its causal effect on the dependent variable." (p. 25) Perhaps an example would help clarify things. A group of researchers at Yale University did an experiment that was published in the Journal of Pediatric Research (Oct. '95) looking at the effect of a glucose rich meal on attention deficit hyperactivity disorder (ADHD) children and children without the disorder. They gave the children a glucose rich meal (glucose is a simple sugar that the brain needs to function) and then monitored the children. They found that within three hours the glucose and insulin levels dropped in both groups - as expected. (Insulin moves glucose form circulation system into the cells.) The neurotransmitters (more on neurotransmitters below) epinephrine and norepinephrine now kick in to step up glucose entry into the brain. The surprising thing was that in the ADHD children, the rise in epinephrine and norepinephrine was about half that of the non-ADHD children. (Note: This is NOT to say that all of ADHD children's problems are caused by sugar.)
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