In an attempt to speed the process and to allow better analysis within and outside the firm, many investors and managers use ratio analysis. Ratio analysis useful because it controls for size differences and can be used to quickly spot trends, strengths, or weaknesses. Ratio analysis is generally broken down into one of several catergories. The Du Pont analysis can be used to tie these together. The key thing here is to be careful interpreting any ratio.
Ratios are only useful when looking for trends or when making comparisions of like firms.