Last Value Carried Forward Imputation

This example uses the data set MI_TRIAL.MDD (located in the SAMPLES subdirectory).

Define Longitudinal Variables

Since LVCF can only be performed on longitudinal variables in SOLAS™, our first step will be to define the Longitudinal Variables in the data set.

1. Open the file MI_TRIAL.MDD.
2. From the Variables menu, select Define Variables > Longitudinal…
3. The system assigns the default name LVar1 to the first longitudinal variable. Just type MeasA into the Name field to replace the default name.
4. Click on the variable name in the Elements listbox to enable the Initialize From Variable Name button, then press this button to include all the “MeasA” variables in the Elements in Variable field.
5. The system automatically assigns a period value of zero to the first element, and the remaining elements will be assigned period values of 1, 2, etc. You can change these values by typing in new values.

For example, you might want to change the default period values if your repeated measurement were taken at baseline month1, month6, and month8, i.e. at unequal time intervals. By setting the period values to 1, 6, and 8, you will ensure that linear interpolation of bounded missings will be correct. Here the measurements were taken at month1, month2, and month3, so the default values do not need be changed.

6. Click on New Variable to define the elements of our second longitudinal variable.
7. A dialog box appears, asking if you want to save your changes to the longitudinal variable MeasA. Click Yes.
8. Type the name MeasB in the name field, then click on the variable name in the Elements listbox to enable the Initialize From Variable Name button, then press this button to include all the “MeasB” variables in the Elements in Variable field.
9. When you are satisfied that you have defined your longitudinal variable correctly, click OK to finish.

1. To perform LVCF Imputation, choose Single Imputation > Last Value Carried Forward from the datasheet Analyze menu.
2. The two longitudinal variables that we created appear in the Longitudinal Variables list. Drag and drop the variables MeasA and MeasB from the Longitudinal Variables list into the Variables to Impute field.
3. When you are satisfied with your choice, click OK. The imputed data set is displayed with the imputed values appearing in Blue/Grey.

The value from the last observed period is carried forward to fill in for missing values in later periods. For example; case #7 has a baseline value of 147 for MeasA, but is missing for all subsequent periods. This value of 147 is carried forward to fill in for these missing periods. This imputed data set can be saved for analysis later, or exported to any other statistics package.

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