I would suggest creating a parameter that provides the values that you are looking to change as part of the 'what-if' analysis. Then use this parameter in a calculated field and use the field in your visualizations. If you now show the parameter control and change it, the viz should get updated to reflect the effect of the parameter values chosen.
Does that help?
Thanks Viraj. This is exactly what I've done already, but by doing this the parameter control changes the field values of the entire record set. I need a method in tableau that can control the values at the record level instead of the field level as there are instances in what if scenarios that requires us to chane the raw input or independent variable instead of the constant. The examples i've seen so far only applies parameter controls on constants. In my attachment, I need a control that will change GRP of period 1 independently from period 2 or other periods.
I am guessing it is not really what you want to do, but you should be able to use separate parameters for the periods and an IF/ELSE clause. Usually, it would probably be easier to write your assumptions into your data source and refresh, but I guess this may not always be an option.