Page 72 - Statistics for Dummies

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Part I: Vital Statistics about Statistics
and 14). Because statisticians know so much about the normal distribution
(see the preceding section), these analyses are much easier. See Chapter 11
for more on the Central Limit Theorem, known by statisticians as the “Crown
jewel in the field of all statistics.” (Should you even bother to tell them to get
a life?)
z-values
If a data set has a normal distribution, and you standardize all the data to
obtain standard scores, those standard scores are called z-values. All z-values
have what is known as a standard normal distribution (or Z-distribution). The
standard normal distribution is a special normal distribution with a mean equal
to 0 and a standard deviation equal to 1.
The standard normal distribution is useful for examining the data and deter-
mining statistics like percentiles, or the percentage of the data falling between
two values. So if researchers determine that the data have a normal distribu-
tion, they usually first standardize the data (by converting each data point
into a z-value) and then use the standard normal distribution to explore and
discuss the data in more detail. See Chapter 9 for more details on z-values.
Experiments
An experiment is a study that imposes a treatment (or control) to the sub-
jects (participants), controls their environment (for example, restricting their
diets, giving them certain dosage levels of a drug or placebo, or asking them
to stay awake for a prescribed period of time), and records the responses.
The purpose of most experiments is to pinpoint a cause-and-effect relation-
ship between two factors (such as alcohol consumption and impaired vision;
or dosage level of a drug and intensity of side effects). Here are some typical
questions that experiments try to answer:
✓ Does taking zinc help reduce the duration of a cold? Some studies show
that it does.
✓ Does the shape and position of your pillow affect how well you sleep at
night? The Emory Spine Center in Atlanta says yes.
✓ Does shoe heel height affect foot comfort? A study done at UCLA says up
to one-inch heels are better than flat soles.
In this section, I discuss some additional definitions of words that you may
hear when someone is talking about experiments. Chapter 17 is entirely dedi-
cated to the subject. For now, just concentrate on basic experiment lingo.

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