I have converted my data to a ratio, do I use the same methods to calculate the SEM?

Hi there

First, post hopefully in the right place - the phrase "elementary" summed up my level of knowledge.

I'm comparing the growth of cells in 2 different types of media. For each media type I have a control group. I want to compare the growth of the cells in each media type, but the controls differ slightly. I am thinking, rather than compare the absolute cell number in each media type directly I should convert the values to a ratio of control to intervention for each media type. Then I would compare the ratios.

However, if I did this I can't find anything to explain whether that affects how I do the statistics. I could use the ratios in the same way that I use the absolute cell numbers e.g. I have three ratios, use those numbers directly in SPSS for statistical testing, and calculate the SEM (STDEV/SQRT(N)) for putting on my graphs.

Or is this considered bad form?