Which statistical tests can I conduct to analyse the trend of series data? I have a dataset that measures students' time spent working on a set of mathematics questions. My dataframe looks a little something like this:




Participant ID
Question 1
Question 2
Question 3




1107
54.2
48.9
45.0


4208
53.1
45.6
40.6




I have times for 20 questions for about 200 students. I have observed an overall decrease in time spent per question, as is shown in the figure below:

I would like to accompany this graph with a statistical measure of negative tendency.
I don't think I should use a correlation statistic as the question number is a categorical variable.
I maybe could do a OLS regression, with X being the question number and y being the time spent per question, but I am not sure how to interpret the result.
What else could I try?

Edit
Since a few people have been asking about the context in which this data was collected, you can read all about it in the study pre-registration https://osf.io/f7zgd

While your question variables are categorical, they could also be treated as ordinal, since they are done in sequence, so there is a natural ordering of the questions. In that case something like Spearman's rho correlation coefficient would be... ok.
