Which tense to describe the analysis on a dataset? It is normal to analyze a dataset in a paper. I am not sure when to use present/past tense to describe how you analyze the data and what your results are. Here I use a short example text to illustrate this:


  We analyze the epidemic spreading dataset in 2008. Let I(t) represent
  the number of individuals that are/were infected at t. I(t) is/was averaged between 1am and 3pm when the spreading is/was stable.  Our
  analysis shows/showed that the epidemic has/had infected 100
  individuals, and most of them are/were in EU. The results show
  that on average each individual has/had three neighbors.


Could you please help decide the tense of the verbs in the above paragraph? It would be good if some explanation is provided.

I ask this, because, I see in many papers, they use the present tense for analysis results of an old data. I am not sure that is correct.
In a case such as this, you need to use a careful mixture of tenses, because sometimes you are speaking about an event which is in the past, and sometimes you are speaking about an observation about that event, which still holds in the present.

Thus, in your example:


  We analyze the epidemic spreading dataset in 2008. Let I(t) represent the number of individuals that were infected at t. Our analysis shows that the epidemic had infected 100 individuals, and most of them were in the EU. The results show that on average each individual has three neighbors.


Notice that the facts about the events that occurred in 2008 are in the past tense, but the analytical results are in the present because the analysis is still showing these facts right now as the reader looks at it (I am assuming the "on average" result in your sample is a general conclusion and not just a fact about those 100 individuals in 2008).