Besides all these challenges to secure appropriate data, there are also difficulties in the actual assessment stage. First are the choices of impact categoriesacidification, global warming, etc.but then are the difficulties about how these categories generate numerical results. Global warming impact, for example, demands that a number of different kinds of gases are translated into CO2 equivalents. An impact category like ecosystem toxicity must also be reduced to some significant indicators or collection of emissions. In many cases, a time factor must be considered, because emissions over a short or a long time frame may have very different impacts. For many kinds of impact assessment categories, collections of indicators must be employed to synthesize an overall impact. And this may be done quite differently in one system or another. The assumptions that must be made both in filling in gaps in data, and in assessing the data for environmental impacts, suggest another difficulty with various LCA tools: their transparency. That is, whenever certain assumptions must be made, it is desirable that these assumptions be visible (or at least accessible) to those who need to know how results were arrived at. In some LCA tools, these assumptions are not made explicit to the user. This is especially important when the results of two different LCAs on a particular product come to different conclusions. Of course, this question of transparency is usually more important in LCA tools intended for use by LCA practitioners than those tools intended for designers and policy-makers which must aggregate and simplify more.