I mentioned theory-dependent 'facts' the other day, and here is a good example using a Brad DeLong post from yesterday. Here is RGDP and the trends DeLong sees:
There is a 'fact' of two cases of RGDP/productivity slowdown. The information transfer (IT) model trend tells a different story with different 'facts' (this graph is the same result from this post):
In this view, there hasn't been any falling productivity, just a trend towards a lower average RGDP growth rate. However, the more fundamental (and therefore better) measure in the IT model is NGDP:
I don't want to leave the impression that I am saying: Ha, ha, DeLong is wrong! Maybe the IT model is wrong (although the NGDP path is pretty excellent, I must say). I just wanted to illustrate how different theoretical models lead to different interpretations of data -- different 'facts'. And different theoretical models lead to different questions (and therefore research). Why has productivity slowed down? is a question you'd ask if you drew the trend lines DeLong drew; it's not a question that follows naturally from the IT model trend.
PS The NGDP path includes a demography slowdown component as changes in the growth rate of the labor force are the primary source of non-monetary changes. See here.