Scotland Bill

Can the Minister explain another point he raised? I am puzzled how it will ever be possible within a reasonable timescale to properly assess whether a measure taken by either the UK Government or the Scottish Government resulted in higher growth and therefore a higher tax rate or the other way round. The Minister must know that most of these matters are in dispute, sometimes for years, because no one can be really sure why a tax take went up or down. There can be a hunch or a feeling, but these things are contested maybe even decades after they happen. Given that this is a settlement that has to fix the grant every year, I am just wondering how you do it.