We have a large set of data (approximates to normal distribution), which we are using to create a set of three ratios (Ra, Rb, Rc), for which we can calculate standard errors. These ratios are then being applied to a fixed survey(x), which has a known standard error, to move the date of the sample to a standard date.

We have to present the errors on our overall count, and this has raised two questions:

1. How should we calculate the standard error for the output after we have multiplied the data together as below?

2. How should the standard error be calculated from this when we add multiple sites (different values for Y) together?

Many thanks for your help.