barrons ap计算机科学,巴朗AP心理学Barrons AP Psychology, 5th Edition-81-120.pdf

巴朗AP心理学Barrons AP Psychology, 5th Edition-81-120

calculate the approximate percentage of scores falling between any given z scores. For instance,

approximately 47.5 percent (95/2) of scores fall between the z scores of 0 and +2 (see Fig. 2.2).

While z scores measure the distance of a score away from the mean, percentiles indicate the

distance of a score from 0. Someone who scores in the 90th percentile on a test has scored better than

90 percent of the people who took the test. Similarly, someone who scores at the 38th percentile

scored better than only 38 percent of the people who took the test. A clear relationship exists between

percentiles and z scores when dealing with the normal curve. Someone who scores at the 50th

percentile has a z score of 0, and someone who scores at the 98th percentile has an approximate z

score of +2.

Correlations

A correlation measures the relationship between two variables. As explained earlier, correlations can

be either positive or negative. If two things are positively correlated, the presence of one thing predicts

the presence of the other. In contrast, a negative correlation means that the presence of one thing

predicts the absence of the other. When no relationship exists between two things, no correlation

exists. As an example, one would suspect that a positive correlation exists between studying and

earning good grades. Conversely, one would suspect that a negative correlation might occur between

cutting classes and earning good grades. Finally, it is likely that there is no correlation between the

number of stuffed animals one owns and earning good grades.

Figure 2.2. The normal distribution.

Correlations may be either strong or weak. The str