When I try to merge two dataset on ID number as Key variable the resulting merged file do not add the new variable in the same row but instead make two rows with the same ID number with the variable from dataset 1 in the first row and the variables from the second dataset in the second row. WhaT do I do wrong?

Which procedure are you using: ADD VARIABLES, ADD FILES, or STAR JOIN? It sounds like you are using ADDVARIABLES. From the CSR

BY specifies one or more key variables that determine the order of cases in the resulting file. When BY is

specified, cases from the input files are interleaved according to their values for the key variables.

Cases in the resulting file are ordered by the values of the key variables. All cases from the first file with

the first value for the key variable are first, followed by all cases from the second file with the same

value, followed by all cases from the third file with the same value, and so forth. These cases are

followed by all cases from the first file with the next value for the key variable, and so on.

Perhaps you need to use ADD VARIABLES (Data > Merge Files > Add Variables