Ratio analysis is an important tool for financial analysis that involves calculating and analyzing relationships between key financial data points from statements. It helps assess a company's liquidity, profitability, solvency, financial stability, and risk. When using ratios, it is important to compare the same company over multiple years, against industry benchmarks, and be aware of factors like accounting differences that could distort comparisons. Key ratios include current ratio, acid test ratio, debt-equity ratio, proprietary ratio, and gross profit ratio.