I'm a cell biologist and a lot of my data is expressed as a fraction (percentage) of the total number of cells in a single experiment in a number of separate categories (generally 5 or so). The number of cells in any given experiment is usually in the hundreds, but the number of individual experiments (replicates) is usually three.
Currently what I'm doing is pooling all the data from the three independent experiments and expressing each category as a percentage e.g. Cat1 70%, Cat2 15%, Cat 3 10%, Cat 4 3%, Cat 5 2%. In the figure legend I state supply the total number of cells and the number of independent experiments they derive from.
However, it would be nice to have some sense of how much those category values vary across the three experiments. A colleague recommended that instead of pooling all the data, I calculate the percentages from each experiment, and then calculate mean/SD/SE from the three percentages in each category (one from each independent experiment).
Is this valid? I seem to remember from school that descriptive statistics are based on the assumption that the data are continuous, and that expressing them as a percentage (or fraction) has already converted them to a discontinuous variable. If it's not valid, is there a better way of doing it? It would be desirable to be able to continue to express the results as a fraction/percentage of the total, as the number of cells per experiment will never be exactly the same.