The lastpage response columns helps you track how far into the survey the participant got. LastPage is only relevant for multi-page surveys:
LastPage blank means participant loaded the first page, and did nothing from there (either have just started the survey or abandoned it without clicking Next).
LastPage = 0 means participant clicked Next, but didn't answer all the mandatory questions, and either abandoned the survey, or they are still working on the first page.
LastPage = -1 means they clicked back from the first page to the welcome page, and abandoned the survey.
LastPage > 0 is the question, or question group page they last completed.
