Rounding in IBM SPSS Data Collection Survey Reporter

IBM® SPSS® Data Collection Survey Reporter performs all calculations using the maximum possible accuracy and only performs rounding immediately before it displays figures in a table. Survey Reporter uses a standard rounding algorithm when it performs rounding. When rounding a real number to an integer, for example, Survey Reporter rounds to the nearest integer, except where the decimal places are exactly 5. In these cases, it rounds to the even integer by default. This means that of the two possible rounded values, the one that has an even number as the last significant digit is returned. For example, 15.25 is rounded to 15.2 rather than 15.3. If you want to instead round half values up, you can set the RoundingOptions property to 1. Refer to the topic "Table Properties" in the Data Collection Developer Library for more information.

Apparent anomalies when you change the accuracy of cell contents that are real numbers can usually be explained by the fact that Survey Reporter performs each rounding calculation separately using the maximum possible accuracy. For example, when you display a weighted count of 51.4999999 with one decimal place, it is shown as 51.5. If you then choose to display it without decimal places, it becomes 51. At first sight, you might think this is incorrect because 51.5 should be rounded to the even number 52. However, Survey Reporter performs each rounding calculation separately from the unrounded value, which in this example is 51.4999999, and the figure of 51 is in fact correct.

During the calculation of a base in a weighted table (for example, from counts for use in a percentage calculation), Survey Reporter uses the maximum possible accuracy of the contributing values. If the base is subsequently displayed in the table, Survey Reporter rounds it to the same number of decimal places as the counts. This means that sometimes a base displayed in a table is not exactly equal to the sum of the counts displayed in the contributing cells. For example, the following table shows the values both before and after rounding of the counts for two cells and the base that Survey Reporter calculates:

CellValue before roundingValue shown in table

When you show row or column percentages in a table, Survey Reporter can optionally manipulate the percentages to eliminate anomalies such as these. You can do this by selecting Adjust rounding so that percentages add up to 100% in the Display tab of the Table Properties dialog box.

Note: IBM® SPSS® Statistics calculates table totals from the rounded values shown in the table. In a corresponding SPSS Statistics table containing the same figures, the total would be shown as 7.