David M. Lane Values of Pearson's Correlation, Variance Sum Law,
Measures of Variability The collection of data involves
measurement. Measurement of some characteristics such as height
and weight are relatively straightforward. The measurement of
psychological attributes such as self esteem can be complex. A
good measurement scale should be both reliable and valid. These
concepts will be discussed in turn. The notion of reliability
revolves around whether you would get at least approximately the
same result if you measure something twice with the same
measurement instrument. A common way to define reliability is
the correlation between parallel forms of a test. Letting "test"
represent a parallel form of the test, the symbol rtest,test is
used to denote the reliability of the test. True Scores and
Error Assume you wish to measure a person's mean response time
to the onset of a stimulus. For simplicity, assume that there is
no learning over tests which, of course, is not really true. The
person is given 1,000