'How to work out difference between scores at date 1 and future dates for same ID SPSS

I have a dataset where each ID has visited a website and recorded their risk level which is coded 0-3. They have then returned to the website at a future date and recorded their risk level again. I want to calculate the difference between each ID's risk level from their first recorded risk level.

For example my dataset looks like this:

ID Timestamp RiskLevel

1  20-Jan-21    2

1  04-Apr-21    2

2  05-Feb-21    1

2  12-Mar-21    2

2  07-May-21    3

3  09-Feb-21    2

3  14-Mar-21    1

3  18-Jun-21    0

And I would like it to look like this:

ID Timestamp RiskLevel DifFromFirstRiskLevel

1  20-Jan-21     2      .

1  04-Apr-21     2      0

2  05-Feb-21     1      .

2  12-Mar-21     2      1

2  07-May-21     3      2

3  09-Feb-21     2      .

3  14-Mar-21     1     -1

3  18-Jun-21     0     -2

What should I do?

spssdifference


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