Understanding Society User Support

Let's go step by step.

does the wave 7 include years 2015-2017? Does this mean that this wave includes three years 2015, 2016, 2017?

Wave 7 includes interviews collected from January 2015 to January 2017, but each household and respondent is interviewed just once during this time. Understanding Society started with a complex sample of 30000 households across the whole UK, therefore interviewing all of them in a shorter period was not feasible.

Does the wave 9 include the years 2017-2019?

In the same way, Wave 9 includes interviews collected from January 2017 to May 2019.

Does this mean that thus wave includes years 2017, 2018, 2019?

I'm not sure I fully understand this question, which wave do you mean? If Wave 9, then yes, Wave 9 data is a collection of interviews collected in these 3 years.

Also, as the wave 7 (2015-2017) and the wave 9 (2017-2019) have an overlapping year, does this mean that I cannot use these waves to make differences of outcome variables between those waves (for example the difference between health status for a child in the period 2015-2017 (wave 9) and the health status of the same child in the period 2013-2015 (wave 7))?

The overlap is for the fieldwork periods, there is of course no overlap at the individual level since the individuals are interviewed at approximately one year intervals. The example data for one person can then look like this (g_ is wave 7, i_ is wave 9, you are looking at the interview dates and the mental health GHQ caseness score and the change in it):

You can calculate change similarly for any other variable available.

Referring to your example, you will be comparing the health status of the same child at the date of the interview in wave 7 and at the date of the interview in wave 9 (for child data collected from parents this will be parent's interview date, check the variable names I provided in the other issue you created; for child data collected in the youth interview this will be intdatd_dv intdatm_dv intdaty_dv).

I’d recommend downloading the data to get familiar with the structure. Also, refer to the User Guide, especially the section on the survey timeline:
https://www.understandingsociety.ac.uk/documentation/mainstage/survey-timeline/

Additionally, the Introduction to Understanding Society Moodle course may be useful:
https://www.understandingsociety.ac.uk/help/training/introduction-to-understanding-society-self-paced-moodle/

If you have further questions, please add them to this thread rather than starting a new one.

Best wishes,
Piotr Marzec
UKHLS User Support