Combining data sets across visits:

One common task in any longitudinal study is to combine data across testing waves.  The sample program is an SPSS script for combining the MALTS zygosity file with the Home Shyness composite measure collected at 14 months, 20 months, and 24 months.  Each file is organized on the basis of individual twin id (TWINID).  To access this sample program, click here.

Reorganizing an individual-level file into a twin-paired family file:

Another common task with twin data is to take individual twin data and create a file with paired twin 1 and twin 2 data.  The sample program is an SPSS script file which takes the combined shyness file created above, splits the file into a twin1 sample and a twin2 sample, renames the twin2 variables by changing the first two letters in the variable prefix (V1 --> W1, V2 --> W2, Y2 --> Z2), and then merges the data back together into a family-organized file.  Simple Pearson correlations are run between the twin1 variables and the twin 2 variables.  To access this sample program, click here.

Creating a double-entered version of the twin-paired family file:

When numbering of twins is arbitrary, it is often useful to get an estimate of the twin intraclass correlation by using a double-entry procedure, in which each individual twin exists once in the file as Twin 1, and once as Twin 2.  The sample program is an SPSS script file which takes the twin-paired family file from the preceding example and adds in a mirrored file in which Twin 1 variables have been renamed as Twin 2 variables, and Twin 2 variables have been renamed as Twin 1 variables.  Degrees of freedom have been adjusted by weighting each case (family) by 0.5.  Twin intraclass correlations have been estimated for MZ and DZ twins.  To access this sample program, click here.