In the year 1998, a survey was undertaken to find the salary of employees working in software companies. In a sample of 450 employees, 25% of them received a salary of $4000 per month. A similar survey was conducted three years later and showed that 15% of employees received $4000 per month in a sample of 600 employees. Construct a 99% confidence interval for the difference in population proportions of employees whose salary was $4000 per month in 1998 and employees whose salary was $4000 per month three years later. Assume that random samples are obtained and the samples are independent. (Round your answers to three decimal places.)