## Wednesday, April 3, 2013

### Visual Education Statistics - Standard Deviation

4
Statistic Three:  The standard deviation (SD) is an attempt to capture the distribution of scores (in a standardized way; visually, the normal curve in the prior post). The 22 scores in the Nursing124 data (Tables 2 and 3) have been sorted only to make the charts easier to follow (Chart  10).

[Check Table 4 below, (PUP 5.20, Table 3c. Guttman Mark Matrix with Scores and Item Difficulty), for the values being plotted.]

[A Guttman table has the student scores sorted from high to low, vertically, and item difficulties sorted high to low, horizontally. The most difficult item and the lowest scoring student end up at the lower right corner of the table.]

The variation in the student scores is readily visible when the average score is added to the chart (Chart 11). It is this region of variation that is captured by the standard deviation (SD). You can add or subtract a constant number from every student score without changing the variation.

The deviation of each score from the average test score (the mean) is extracted from the scores and is plotted next (Chart 12). These values add to zero. The solution to this problem (back when this was all done with paper and pencil) was to square the deviations. Now the numbers being used to capture the variation in the scores are all positive and can be added (Chart 13). The sum of squares (SS) is 89.86.

About ½ of the SS is produced by just 3 of the most extreme scores out of the 22 total. It is a matter of personal judgment when to call an extreme score an outlier and remove it from further statistical analysis.

The calculation of the SD involves three steps shown on the right side of the Guttman table (Table 4): the sum of squared deviations (SS) [89.86], the mean of the sum of squares (Mean SS, MSS or Variance) [4.28], and the SD (Square Root of the Mean SS or the Variance) [2.07]. Each step has been given a name as in many calculations the SS or the Mean SS is used rather than developing the SD and then reversing the process as needed.

Standard here means done in a standardized way. The standard is that 2/3 (68.3%) of the student scores and item difficulties are expected to fall within the range of (+-) one SD of their average values. And 95.4% are expected to fall within the range of (+-) two SDs of their average values.

This standard generates the normal curve of error that in education is shortened to the normal curve or bell curve with both the meaning and use reversed. In the sciences and engineering, error is to be avoided. In education, error is used to produce the spread of scores needed to assign letter grades in schools designed for failure and to assign the pass/fail point on NCLB standardized assessments.

[This need for a wide distribution of scores for setting test grades may result in problems in establishing the precision of student scores. This needs to be checked when I get to the standard error of measurement, statistic #5, in this series.]

The above calculations are shown for N and for N – 1 when calculating the Mean SS. N – 1 is a correction for classroom sized test data, when the number of students and questions is below 100.

Table 4 shows the SD for student scores and for item difficulties. Item difficulties generally have a wider spread (3.17), a larger SD, than student scores (2.07) in classroom tests.

I have fully developed Table 4, Standard Deviation (SD) Calculations, as this shows the foundation for all of the remaining statistics in this series. The sum of squared deviations (SS) is 89.86 for student scores and 201.40 for item difficulties. Then dividing SS by the number summed produces the Mean SS or Variance. The square root of the Mean SS is the SD. This is totally reversible. Square the SD yields the Mean SS. Multiply the Mean SS by the number of summed to get back to the SS.

The above paragraph reports honest number manipulations. There is no room for bias in calculating the SS. However, the deviations squared (DEVSQR) are spread over a wider range than the actual deviations. A large SD can be due to an evenly distributed set of scores or to a narrow distribution with one or more outliers far from the average score. Two identical SDs may result from two very different distributions.

This can be a problem in classroom tests. Standardized tests reduce the problem by sampling a large enough number of students or items to get a stable distribution.

[The SS are also used in the analysis of variance (ANOVA) commonly used in the sciences and engineering. I never saw it used in education when I was teaching. The ANOVA permits one to determine if the distribution of marks in rows and columns is just a matter of luck or if there is something else at play. If the null hypothesis, that the distribution is no different than a matter of luck, holds, there is then no need to do any other statistical tests.

The calculations (in yellow) for the ANOVA have been added to the left and bottom edge of Table 4. The grand mean is 0.779 (based on the values of right and wrong marks, 1 and 0). The deviations squared (DEVSQR) for each score and difficulty are listed in respect to the grand mean. The total degrees of freedom is the count of cells in the table minus 1 (462 – 1).

The ANOVA (Table 5) yields Mean SS ratio between student score rows and unexplained (or error) within rows or between columns (0.20/0.16) for an F test value of 1.27. This value does not exceed the 5% level of significance F table for 21/440 degrees of freedom critical value of 1.59. The variation found in this table of student marks may be a matter of luck (student preparation and attitude, item authoring, test creator item selection, testing environment, and pure chance).]

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