Computing Test for Independent Samples in the Sheets tab
Tests for independent samples include the most common statistical tests for comparing values:
of a continuous attribute X in two groups, defined by a binary attribute Y, or
of two continuous attributes X, Y.
Number of valid positive, negative and total samples
The number of valid samples for both attributes n is displayed, for positive, negative and total data samples respectively.
This is particularly useful when there is a heavily unbalanced distribution of missing data among the two attributes, which might cause the analysis to be based on an unacceptably small sample size.
Wilcoxon and Mann-Whitney test
The non-parametric test of Mann-Whitney and the Wilcoxon equivalent variant are both available.
Items include U-value, which represents the number of couples of subjects selected by the two groups among which the continuous attribute shows the highest value in the target group. In the U calculation, couples with equal values contribute with 0.5 each.
The Normalized U-value is the corresponding proportion of couples, thus representing an estimate of the probability of correctly ranking two subjects randomly selected from the two groups. Normalized U-value is the equivalent to the AUC area under the ROC curve obtained by the two selected attributes.
R1-value is the lowest value of the rank sum, calculated inside the two groups. It frequently, but not necessarily, corresponds to the sum of ranks inside the smallest group. R1 is also known as the Wilcoxon statistic test and is easily obtained by the following procedure:
Under the null hypothesis of an equal distribution of the continuous attribute inside the two groups, the two corresponding rank sums will be similar, then a very low value of R1 will correspond to a high probability of an actual difference between the two distributions. In the absence of coincident values (ties) the Mann-Whitney test and the Wilcoxon test are equivalent to assess the difference between the median values among the two classes. The test is very powerful in the presence of a “shift” model, i.e. when the two groups share distributions that differ only by a constant value. Conversely, the test in general is unable to identify small local differences between the two distributions (the Kolmogorv-Smirnov test could be applied instead).
The equivalence between the Mann-Whitney test and the method proposed by Wilcoxon relies on the fact that U is a simple linear transformation of R1:
where n1 and n2 represent the sample size of the two groups. The corresponding p-value is obtained by assuming an asymptotic Normal distribution for the U statistic:
where z follows a Normal standard distribution.
KS Value and P-value for KS test
A non-parametric tool for comparing a continuous distribution between two groups.
Similarly to the Wilcoxon-Mann-Whitney approach it tests the null hypothesis that the two distributions are equal, looking for any difference inside the two empirical distributions. In particular, in the presence of local differences it tends to be more powerful than the Mann-Whitney test, whereas when the continuous attribute has a similar distribution among the two groups, but with different median values, the former approach is uniformly more powerful.
The Kolmogorov-Smirnov test is based on the maximum absolute difference between the two empirical distributions Sn1 and Sn2 of the values of continuous attribute inside the two groups:
The corresponding p-value is obtained by the exact cumulative null distribution of n1n2Dn1,n2. The Kolmogorv-Smirnov statistic Dn1,n2 has an interesting relationship with the ROC curve obtained from the same two attributes, as it corresponds to the highest vertical distance between the curve and the rising diagonal, also corresponding to the highest value of the Youden index.
Student t-value and P-value for Student t-test
The most popular statistical test for comparing two classes.
It is used to assess the null hypothesis that the mean values of the distribution of the continuous attribute inside the two groups are not different. It lies on the assumption that the continuous variable is normally distributed inside each group.
The test is available in two different versions. The first default test assumes that the continuous variable has the same variance in the two groups:
where m1 and m2 represent the means of the two groups, S1 and S2 the corresponding variance estimates and n1 and n2 the number of valid data in each group. The expression inside the square root at the denominator of the equation is usually called “the estimate of the pooled variance”.
The Student t test for unequal variances is obtained by the following equation:
In both cases the inference (i.e. the p-value calculation) is made by assuming that t follows a Student t distribution with n1+n2−2 degrees of freedom. Such an assumption is met when the two samples are independent and the distribution of the continuous attribute within each group is Normal (Gaussian).
F-value for Levene test and P-value for Levene test
This test is used to compare the variance of the continuous attribute between two groups, using the following equation:
F follows a Fisher F distribution when the two variances are actually equal (i.e. under the null hypothesis) and if there is a Normal distribution of the attribute inside the two groups. The degrees of freedom for the numerator and the denominator corresponds to n2−1 and n1−1 respectively, if s22 > s21.
If however s21 <= s22 the degrees of freedom for the numerator and the denominator instead correspond to n1-1 and n2-1, respectively.
Use target attribute
Selecting target value for … it is possible to choose the target value for each statistical analysis.
If the target attribute is dichotomic, the selection will have an effect on the Student t test only, changing the sign of the t statistic; because the corresponding p-value is evaluated for two sided tests, it will remain unchanged.
Student t-test parameters
The type of Student t test.
Possible types are: