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Outline
Continue
the discussion on interpreting r
Other
types of correlations
Spearman
Point-Biserial
Phi-Coefficient
SPEARMANT
RANK-ORDER CORRELATION (RHO)
Index
of correlation between two variables measured along an ordinal scale.
It is also used with data that result in a nonlinear relationship, as
long as the original scores are converted to ranks.
The Pearson Product-Moment formula is then used to compute the
correlation but the symbol used is rs.
Figure 16-14
(p. 542)
Hypothetical
data showing the relationship between practice and performance. Although this
relationship is not linear, there is a consistent positive relationship. An
increase in performance tends to accompany an increase in practice.
For example, consider the following data
The ranks will be used in order to compute the correlation coefficient.
POINT-BISERIAL
CORRELATION
Index
of correlation between two variables, where one is measured on an interval or
ratio scale, but the other variable is dichotomous. A dichotomous variable has only two values (e.g. male vs.
female).
To
compute the point-biserial correlation, the dichotomous variable is first
converted to numerical values by assigning a value of 0 to one category and a
value of 1 to the other. Then the
regular Pearson correlation formula is used with the converted data.
PHI
COEFFICIENT (f)
Index
of correlation between two variables measured along a nominal scale (so both are
dichotomous). Two steps are required in the computations.
1.
Convert
each of the dichotomous variables to numerical values by assigning 0 to one
category and 1 to the other.
2.
Use the
regular Pearson formula with the converted scores.