Assuming that's the typical error I would do this:
I would define my limits to the ROI, let's say between -0.5 and 2 (-50% and 200%) then I would create an Updated ROI field in the loading script that, if within the limits, the same value but if not, the value / 100. At the same time I'd create a flag as UpdatedROI_Flag with 0 for the ones that you didn't update and 1 for the ones that you updated, so i'ts easy to see and troubleshoot what you did.
This of course assumes you don't have have projects with 2500% ROI... well, those would be amasing projects!!!
You could also create it on the chart after the loading script but it'll depend on the size of your data. I usually don't like to create chart fields with IFbut it could work... I'm talking Qlik Sense in this part. I've never see Qlik View
I hope this helps,
Just wondering to know how you went about this one, Did you manage to get some control upstream? or maybe a report to users to update accordingly?
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You could use the PurgeChar() function in your load script to remove the % symbols
Maybe some like :
num ( PurgeChar ( [ROI] , '%' ) / 100 ) , '#o%' )
That is assuming they both have same numeric interpretation. If 25% is otherwise represented as 0.25 then you will probably need to use an If statement using the Index() function to only divide by 100 if a % character is found.