In SPSS, why do the SUM() and MEAN() functions keep cases with missing values instead of dropping those observations?

Statistical functions in SPSS [e.g., SUM(), MEAN(), SD()] perform calculations using all available cases. SPSS will not automatically drop observations with missing values, but instead it will exclude cases with missing values from the calculations. SPSS will correctly estimate the mean with the MEAN() function by using all non-missing values.

However, problems can arise when trying to exclude missing cases and estimate results based only on observations with complete information. For example, suppose two variables (v1 and v2) sum to create an index variable (v3). While v1 has ten valid cases with no missing values, v2 has eight valid cases and two missing values. Use the following syntax to add the two variables and create an index, v3:

  COMPUTE V3 = SUM(V1, V2).
  EXECUTE .

The resulting index variable v3 has ten cases and no missing values. When SPSS encounters a missing value in any of the v2 cases, it ignores it and sets v3 equal to v1. Essentially, SPSS treats the missing values of v2 as zeroes. The results can potentially be misleading.

To ensure that v3 is equal to the sum of v1 and v2 and that all missing cases are dropped rather than ignored, specify the minimum number of valid cases that SPSS should use to calculate a given function. For example, to create an index variable v3 using only observations without missing values, execute the following syntax:

  COMPUTE V3 = SUM.2(V1, V2).
  EXECUTE .

The .2 appended to the end of the SUM function in the above example can be any integer. Use it to indicate the minimum number of valid cases necessary to perform a given calculation.

If you have questions about using statistical and mathematical software at Indiana University, contact Research Analytics. Research Analytics is located on the IU Bloomington campus at Woodburn Hall 200; staff are available for consultation Monday-Friday 9am-noon and by appointment.

This is document adjn in the Knowledge Base.
Last modified on 2015-06-22 00:00:00.

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