Data Mining by Mehmed Kantardzic (inspirational novels TXT) 📗

can be computationally very complex, and a modified heuristic procedure is used to produce a near-optimum solution. The procedure consists of the following steps:

1. Sort all values for a given feature.

2. Assign approximately equal numbers of sorted adjacent values (vi) to each bin, where the number of bins is given in advance.

3. Move a border element vi from one bin to the next (or previous) when that reduces the global distance error (ER; the sum of all distances from each vi to the mean or mode of its assigned bin).

A simple example of the dynamic bin procedure for feature discretization is given next. The set of values for a feature f is {5, 1, 8, 2, 2, 9, 2, 1, 8, 6}. Split them into three bins (k = 3), where the bins will be represented by their modes. The computations in the first iteration of the algorithm are

1. Sorted set of values for feature f is {1, 1, 2, 2, 2, 5, 6, 8, 8, 9}

2. Initial bins (k = 3) are

3. (i) Modes for the three selected bins are {1, 2, 8}. After initial distribution, the total error, using absolute distance for modes, is

4. (iv) After moving two elements from BIN2 into BIN1 and one element from BIN3 to BIN2 in the next three iterations and obtaining smaller and smaller ER, the new and final distribution of elements will be

The corresponding modes are {2, 5, 8}, and the total minimized error ER is 4. Any additional move of elements between bins will cause an increase in the ER value. The final distribution, with medians as representatives, will be the solution for a given value-reduction problem.

Another very simple method of smoothing feature values is number approximation by rounding. Rounding is a natural operation for humans; it is also natural for a computer, and it involves very few operations. First, numbers with decimals are converted to integers prior to rounding. After rounding, the number is divided by the same constant. Formally, these steps are described with the following computations applied to the feature value X:

1. Integer division: Y = int (X/10k)

2. Rounding: If (mod [X,10k] ≥ [10k/2]) then Y = Y + 1

3. Integer multiplication: X = Y * 10k

where k is the number of rightmost decimal places to round. For example, the number 1453 is rounded to 1450 if k = 1, rounded to 1500 if k = 2, and rounded to 1000 if k = 3.

Given a number of values for a feature as an input parameter of the procedure, this simple rounding algorithm can be applied in iterations to reduce these values in large data sets. First, a feature’s values are sorted so that the number of distinct values after rounding can be counted. Starting at k = 1, rounding is performed for all values, and the number of distinct values counted. Parameter k is increased in the next iteration until the number of values in the sorted list is reduced to less than the allowable maximum, typically in real-world applications between 50 and 100.

3.7 FEATURE DISCRETIZATION: CHIMERGE TECHNIQUE

ChiMerge is one automated discretization algorithm that analyzes the quality of multiple intervals for a given feature by using χ2 statistics. The algorithm determines similarities between distributions of data in two adjacent intervals based on output classification of samples. If the conclusion of the χ2 test is that the output class is independent of the feature’s intervals, then the intervals should be merged; otherwise, it indicates that the difference between intervals is statistically significant, and no merger will be performed.

ChiMerge algorithm consists of three basic steps for discretization:

1. Sort the data for the given feature in ascending order.

2. Define initial intervals so that every value of the feature is in a separate interval.

3. Repeat until no χ2 of any two adjacent intervals is less then threshold value.

After each merger, χ2 tests for the remaining intervals are calculated, and two adjacent features with the χ2 values are found. If the calculated χ2 is less than the lowest threshold, merge these intervals. If no merge is possible, and the number of intervals is greater than the user-defined maximum, increase the threshold value.

The χ2 test or the contingency table test is used in the methodology for determining the independence of two adjacent intervals. When the data are summarized in a contingency table (its form is represented in Table 3.5), the χ2 test is given by the formula:

where k = the number of classes,Aij = the number of instances in the ith interval, jth class, Eij = the expected frequency of Aij , which is computed as (Ri Cj)/N,            Ri = the number of instances in the ith interval = ∑ Aij, j = 1,…, k, Cj = the number of instances in the jth class = ∑ Aij, i = 1,2, N = the total number of instances = ∑ Ri, i = 1,2.

If either Ri or Cj in the contingency table is 0, Eij is set to a small value, for example, Eij = 0.1. The reason for this modification is to avoid very small values in the denominator of the test. The degree of freedom parameter of the χ2 test is for one less than the number of classes. When more than one feature has to be discretized, a threshold for the maximum number of intervals and a confidence interval for the χ2 test should be defined separately for each feature. If the number of intervals exceeds the maximum, the algorithm ChiMerge may continue with a new, reduced value for confidence.

For a classification problem with two classes (k = 2), where the merging of two intervals is analyzed, the contingency table for 2 × 2 has the form given in Table 3.5. A11 represents the number of samples in the first interval belonging to the first class; A12 is the number of samples in the first interval belonging to the second class; A21 is the number