Читать книгу Analysing Quantitative Data - Raymond A Kent - Страница 46

SPSS makes a distinction between two kinds of missing value: system missing values and user-defined missing values. The former result when the person entering the data has no value to enter for a particular variable (for whatever reason) for a particular case. In this situation the data analyst will just skip the cell and SPSS will enter a full stop in that cell to indicate that no value has been recorded. For most non-graphical outputs, SPSS will list in a separate Case Processing Summary the number of valid and the number of missing cases. In some tables, as in Figure 2.5 in an earlier section, valid and missing cases are shown in the printed output table itself. Percentages are then calculated both for the total number of cases entered into the data matrix and for the total of non-missing cases for that variable – what SPSS calls the Valid Percent.

User-defined missing values are ones that have been entered into the data matrix, but the researcher decides to exclude them from the analysis. To create them for any particular variable, from the Variable View select the little blue box in the Missing column against the variable you want and obtain the Missing Values dialog box. This enables you either to pick out particular codes to be treated as missing values by clicking on the Discrete missing values radio button and entering up to three codes, or to select a range of missing values.