Page 519 - Encyclopedia of Nursing Research

486 n STATISTICAl TeChnIQUeS

does regression or discriminant function tetrachoric coefficients are estimates of r,
analysis and provides estimates in terms of given certain conditions. True nonparametric
S odds ratios that add to the understanding of measures of relationship include Kendall’s
the results. tau and the contingency coefficient. Kendall’s
Chi-square is the most frequently tau was developed as an alternative proce-
reported nonparametric technique. It is used dure for Spearman rho. It may be used when
to compare the actual number (or frequency) measuring the relation between two ranked
in each group with the “expected” number. (ordinal) variables. The contingency coef-
The expected number can be based on theory, ficient can be used to measure the relation-
previous experience, or comparison groups. ship between two nominal-level variables.
Chi-square tests whether or not the expected The calculation of this coefficient is based on
number differs significantly from the actual the chi-square statistic. nonparametric tech-
number. Chi-square is the appropriate tech- niques should be considered if assumptions
nique when variables are measured at the about the normal distribution of variables
nominal level. It may be used with two or cannot be met. These techniques, although
more mutually exclusive groups. When the less powerful, provide a more accurate
groups are not mutually exclusive, as when appraisal of group differences and relation-
the same subjects are measured twice, an ships among variables when the assump-
adaptation of chi-square, the mcnemar test, tions underlying the parametric techniques
may be appropriate. The mcnemar test can be have been violated.
used to measure change when there are two Regression is a statistical method that
dichotomous measures on the subjects. When makes use of the correlation between two
comparing groups of subjects on ordinal variables and the notion of a straight line to
data, two commonly used techniques are the develop an equation that can be used to pre-
mann–Whitney U, which is used to compare dict the score of one of the variables, given
two groups and is thus analogous to the t test, the score of the other. In the case of a multiple
and the Kruskal–Wallis H, which is used to correlation, regression is used to establish a
compare two or more groups and is thus anal- prediction equation in which the indepen-
ogous to the parametric technique AnOVA. dent variables are each assigned a weight
When one has repeated measures on two based on their relationship to the dependent
or more groups and the outcome measure is variable, while controlling for the other inde-
not appropriate for parametric techniques, pendent variables.
two nonparametric techniques that may be Regression is useful as a flexible tech-
appropriate are the Wilcoxon matched-pairs nique that allows prediction and explanation
signed rank test and the Friedman matched of the interrelationships among variables
samples. The Wilcoxon matched-pairs is and the use of categorical as well as contin-
analogous to the parametric paired t test, and uous variables. Regression literally means a
the Friedman matched samples is analogous falling back toward the mean. With perfect
to a repeated-measures AnOVA. correlations, there is no falling back; using
In addition to nonparametric tech- standardized scores, the predicted score
niques for making group comparisons, there is the same as the predictor. With less than
are nonparametric techniques for measur- perfect correlations there is some error in
ing relationships. There is some confusion the measurement; the more error, the more
about these techniques. For example, point- regression toward the mean.
biserial and Spearman rho are often con- In multiple regression, the multiple cor-
sidered nonparametric techniques but are relation (R) and each of the b-weights are
actually shortcut formulas for the pearson tested for significance. In most reports, the
product–moment correlation (r). Biserial and squared multiple correlation, R , is reported,
2