Welcome to Science Buddies and thank you for your question. This one can be a bit tricky when identifying the Independent (IV), Dependent (DV), and Control Variables (CV). It seems at first look that since you are varying the salt levels that solution concentration would be the IV. Really what you are doing is changing the age of the test subject and looking for a relationship with their age and the ability to detect salt. So your IV is the age of the test subject, your DV is the ability to detect salt, and your CV is the salt concentration of the solutions.
For the CV, even though you have three (or more) solutions, this is the conditions of the test under which the test subjects will be subjected. You will control them by ensuring each test subject gets exactly the same concentrations. I actually see that you have at least 3 CVs.
CV#1) Salt concentrations (0%, 0.05%, 0.10%) per the project proceedures. More if you decide to modify it, but I wouldn't do less. You're looking for the level at which someone detects salt in the solution.
CV#2) Fluid amount. You should ensure each participant gets exactly the same amount of all 3 solutions. For example, don't provide 1oz of 0% control and then 5oz of 0.10% solution to ensure the amount they drink doesn't have an influence on an individuals ability to detect.
CV#3) Temperature of fluid. I would ensure the temperature of each sample solution is as close to the same in all cases. For example, don't give one solution sample at room temp and another right out of the refrigerator to ensure this isn't having an effect on the ability to detect.
As for a better rating system. In this case, you're looking for the ability to detect salt. Either the subject does or doesn't. If you wanted to change the proceedures you could try to have your test subjects rate the level of saltyness from 0-9 or 1-10 or something, but this gets really subjective and would require alot of participants in each age group. For example, a salty level 5 to me may only be a 3 to someone else of the same age. If you want to determine with more certainty at what point someone of a certain age really detects salt, you could add different concentrations of solutions (i.e. 0, 0.02, 0.05, 0.07, 0.10), but leave the rest of the proceedures the same. Additionally, you could divide your test groups into more age groups, like <=20, 21-40, 41-60, >60, just as an example.
I hope this helps.
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