I realized I was doing lots of threads on the same topic so I decided to consolidate my questions here.
I have read time series for years (I am not a statistician, I have graduate degrees that involve statistic, but effectively no knowledge of calculus or matrices). Certain questions continue to elude me in part because various writers on sites and books disagree with each other. Here is a very basic, but important question. When do time series concerns apply and when do they not? That is when must you do time series and when can you use cross sectional approaches?
I used to think that this was simple, if X occurs before Y than you do time series. But of course X always occurs before Y if it influences Y. We answer two types of questions. First, those where the results are measured at one or two distinct points in time (six or 12 months after a customer leaves). For us, spending influences (we assume) results, so types and quantity of spending is the predictor and say income gain is the result. But spending can take place at many, many points in time, it is rare for it to occur at just one point (and we are not always certain when it is actually provided just that it has occurred in the past). And the result takes places years after the spending with a customer getting many types of spending over time. Further, one case can have the spending at say a year before the result is measured another six months and so on.
For a second type of issue we deal with we need to know how spending each month is influenced by various factors. We have no theory, I have found none, to build predictions on. We are a government agency and the literature I have found contains no time series analysis.
Given the much greater complexity and judgement called for by all time series methods I know (for example VAR/VECM] I would always prefer cross sectional to time series methods - particularly since the later generally make it impossible to include the many control variables I feel are necessary.